Research in cognitive psychology has indicated that alternative descriptions of the same event can give rise to different probability judgments. This observation has led to the de velopment of a descriptive account, called support theory, which assumes that the judged probability of an explicit description of an event (that lists specific possibilities) generally exceeds the judged probability of an implicit description of the same event (that does not mention specific possibilities). To investigate this assumption in medical judgment, the au thors presented physicians with brief clinical scenarios describing individual patients and elicited diagnostic and prognostic probability judgments. The results showed that the phy sicians tended to discount unspecified possibilities, as predicted by support theory. The authors suggest that an awareness of the discrepancy between intuitive judgments and the laws of chance may provide opportunities for improving medical decision making. Key words: probability judgment; support theory; unpacking principle; cognition. (Med Decis Making 1995;15:227-230)