Verbal probability terms are frequently used in medical practice. In the present experiment the use of verbal and numerical probability terms in medical decisions was investigated. Interns, residents in surgery and internal medicine, surgeons, and internists were asked to make treatment decisions for three different cases (acute appendicitis, angina pectoris, and an imaginary disease) and were also asked to give numerical interpretations of a series of verbal probability terms. In the second stage of the experiment the respondents received the same cases, but with numerical probability terms. The results showed no effect of context or of domain experience on the interpretation of verbal terms. Residents and experienced surgeons more often agreed on treatment decisions when chance information was presented in numerical terms as compared with verbal terms. Physicians were less confident when verbal terms were presented, but only for the less familiar decision problems. Finally, phy sicians turned out to be better in Bayesian reasoning when numerical terms were used. Experienced physicians were quite accurate in estimating the posterior probability in the appendicitis case, but not in the imaginary-disease case. Key words: verbal probabilities; Bayesian reasoning; expertise. (Med Decis Making 1994;14:146-156).