In the differential diagnosis of patients in whom Cushing's syndrome is suspected, the physician uses clinical signs and simple laboratory data in addition to information gained from past experiences to make a decision concerning the probability of the diagnosis and the need for further investigation. He usually does not make an explicit formulation of either the information or the reasoning process leading to his decision. This report is an examination of an explicit method for estimating the probability of Cushing's syndrome in patients in whom this diagnosis is suspected, using clinical signs and simple laboratory techniques, not including steroid analyses. The clinical and nonsteroid laboratory data of 211 patients investigated for Cushing's syndrome were examined to determine the incidence of the signs of the syndrome in the patients with and without the disorder. These data were used to calculate the probability of Cushing's syndrome in 111 additional patients examined because Cushing's syndrome was...