Qualitative expressions of probability, such as "likely," have different numerical meanings to different people, which can lead to misunderstanding among physicians and between physicians and patients. In a study conducted through a nationwide interactive computer network based at Massachusetts General Hospital, we gathered information on the meaning of common expressions of probability. Three groups of medical professionals assigned percentage values to 12 expressions of the probability that a given symptom would appear in a patient with an unspecified disease. The median values assigned to these expressions by physicians, medical students, and other professionals were almost the same. Comparisons of the means for 7 of these 12 expressions with those found in an earlier study by other investigators showed that they were quantified in the same order, although they had not been assigned the same numerical values. This degree of agreement among professionals and between studies is encouraging for the future prospects of codifying the meaning of such expressions. The variation among five studies in the mean values assigned to 37 expressions in the medical literature and the variation among individual opinions show that such codification is necessary. In the meantime, the average numerical values presented here for various qualitative expressions of probability could well be used to enhance communication among medical professionals.