The process of formal decision analysis is illustrated by using the case of the hepatitis B vaccine in a computer program designed to teach both subjects to medical students. The software presents the medical facts, determines personal risks, allows input of personal values and calculates utilities for the various branches of the decision tree. There are three possible choices: vaccinate; screen first for antibodies against the hepatitis B virus and vaccinate only if negative; and wait (do not vaccinate). The decision with the utility of the highest numerical value is assumed to be the best under the given set of circumstances. Unlike commercial packages, this software contains the information internally and does not require that the user be familiar with either medical decision analysis or with the medical aspects of hepatitis. The program is available for PC's, Medical decision Analysis Hepatitis B Risks Values Utilities Pascal Teaching medical decision analysis Wait Screen No hepatitis, no vaccine complication Hepatitis, no vaccine complicat~an No hepatitis, no vaccine CompLicatlon Hepatitis, no vaccine complication No hepatitis, vaccine complication Hepatitis, vaccine complication HBV+ , Hepatitis, no vaccine complication No complication No hepatitis, no vaccme compticatian HBV-Hepatitis. no vaccine campticatlon About the Author-DAVID R. THOMAS received the M.D. degree in 1971 from the University of Mississippi Medical Center where he is currently an assistant professor of medicine and attending Physician in the Division of General Medicine. His special interest is in the application of medical decision analysis to clinical medicine and health delivery.