In most states, and for most cases, the legal elements of a claim for attorney professional malpractice are:

• (1) an attorney-client relationship existed (The legal malpractice expert witness must state his description of that element  with clarity);
• (2) the attorney acted negligently or in breach of contract (The legal malpractice expert witness must state his description of that element  with clarity).;
• (3) the attorney's acts were the proximate cause of some damages sustained by the client; and
• (4) but for the attorney's negligence, the client would have been successful in prosecuting/defending an underlying claim, or completing the transaction involved.

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The expert witness must show the jury facts, law, and legal ethics.  It must be done clearly, so that the jury and the judge understand what happened, and how it was/was not below the standard, and why it was/was not ethically proper for it to occur.   The jury must understand the verdict that will make things "right".

You must have an expert, and your expert must testify on each of the items, on your side of the case.

Items on which an expert can help follow.  Some of them are items that attorneys and experts overlook as possible expert testimony items

• Malpractice in the underlying case or transaction at issue: whether or not legal malpractice exists in the handling of the underlying case or transaction is the issue between the plaintiff and defendant. This is the usual item that most attorneys think of when they retain an expert witness to explain professional malpractice. But often there is more that can be done by an expert attorney witness in a professional negligence case.
   
• Breach of fiduciary duty.  In some instances, in addition to a duty of due care, there was a separate fiduciary duty.  For example, a conflict of interest may be a breach of fiduciary duty.  To involve/defeat the introduction of jury instructions and the punitive damages that may be involved with fiduciary duty, an expert needs to explain the facts showing fiduciary duty and the breach of duty as something contrary to the acts of a reasonable attorney.  See, e.g., Deutsch v. Hoover et al, 97 SW3d 179 (Tex.App., Houston, 2002) for a discussion of the possibility of both malpractice and breach of duty existing in the same case.
   
• Damages --- what was lost by the defect in the underlying case or transaction: what was lost (not the value of what was lost) is sometimes within the area of expert testimony.  The easy example is the loss of the ability to sue because of the failure to meet a statute of limitations.   The less common example is the legal costs of pursuing/defending underlying rights when the legal cost was occasioned by the negligence
  
  Sometimes the value of litigation that has been lost  is the reasonable settlement value of the underlying litigation case if it had been properly handled. The theory of settlement value as a measure of damages is logical, but it is new.  It also makes a lot common sense, because most cases are settled, not tried to conclusion. This is accepted by some courts as a measure of the damages of the plaintiff  particularly if the claim is that a lawsuit was improperly settled. 

  Traditionally, the measure of damages is not stated in terms of settlement value. For example, in Cosgrove v. Grimes, 774 S.W.2d 662,666 (Tex. 1989),the court said that the amount of damages in a legal malpractice case is measured by "the amount of damages recoverable and collectible from [the defendant in the underlying suit] if the suit had been properly prosecuted."  To determine whether in your case the settlement value could be a measure of damages involves legal research to first determine if such a measure is presently available in your state.  Then, if it is, expert review of the underlying case is necessary to establish the settlement value of the underlying case if it had been reasonably handled.

  Notice that the claimant must also show that "collectability" if the underlying matter had been properly handled.  Sometimes this needs expert explanation, as for example that insurance to make the payment did exist.
   

• Proximate cause of damage from the underlying case or transaction. Where the underlying matter is a transactional matter, the expert witness may be allowed to state his opinion whether but for the negligence the transaction would have been successful.  For example, if the lawyer had included certain language in the will, the distribution of the estate would have been different. 
  
  Where the underlying matter is litigation, the opinion of the expert whether the underlying case would have been won but for the negligence is generally not admitted.  But there are some states and some cases where exceptions allow the testimony. See Whitley v. Chamouris, 574 SE2d 251 (Va. 2003) for an example of how even within a state the rule may shift. In Whitley, in spite of pronouncements in prior cases that :proximate causation in a legal malpractice case...[is] decided by the fact finder after considering testimony of expert witnesses" that a proximate cause opinion would be improper because "no witness can predict the decision of a jury."
  
  And in some states, the "most probable" result rule may become a vehicle for an expert to testify on "probability".  See for example the language in Hall v. Fedor, 561 SE2d 654 (S.C. App. 2002)(On claim of inadequate settlement caused by negligence, plaintiff has to show he would have "most probably" received a larger settlement or "most probably" prevailed at trial of the underlying claim.).
   
• Costs --- if plaintiff wins the malpractice case: the reasonable value of attorney's fees and costs in that part of the malpractice case which involved proving the reasonable value of the underlying case or transaction  may sometimes be recovered (see points # 2 and # 4 at hyperlinked page) as costs by a prevailing plaintiff. Most attorneys, even experts in legal malpractice claims, often are not aware of this element of damages.

Attorneys prosecuting and defending attorney negligence lawsuits may want to Read tips and a list of types of opinions that can be elicited in the court or deposition testimony of a qualified legal malpractice expert.

  Tips on choosing an expert witness.