Informed consent litigation provides a forum in which probabilistic evidence is elicited from physicians as parties or as expert witnesses. The authors reviewed over 450 medical in formed consent opinions reported by both trial and appellate courts in all 50 states over 40 years to determine 1) the extent to which verbal expressions of probability were used by testifying physicians to characterize the risks of medical procedures; 2) when such expres sions were used, whether consistent numeric interpretations of the terms were being applied by the physicians; 3) whether the choice of expression was influenced by the severity of the consequences associated with the particular risk, and 4) whether the use of such terms was correlated with trial outcomes, inasmuch as the duty to disclose a risk is said to increase with the magnitude of the risk and probability is one measure of such magnitude. It was found that subjective verbal expressions of probability are used in the litigation setting, and that such expressions represent broad ranges of numeric probabilities. There was some correlation between the expression and the represented numeric probabilities. In general, expressions such as "extremely low" and "low" corresponded to probabilities lower than those represented by terms such as "high" and "very high." Further, verbal expressions appeared to be influenced by the seventy of the consequences associated with the risks, but whether this increases or decreases the ambiguity of verbal expressions in the com munication process warrants further research. The authors suggest a syntax of verbal expres sions of probability as a means to reduce the numeric ambiguity of these terms Key words: verbal expressions; probability; syntax; communication; medicolegal, litigation; informed con sent. (Med Decis Making 1991;11:273-281)