We prospectively evaluated 91 patients with involuntary weight loss. Thirty-two (35%) had no identifiable physical cause of weight loss, whereas the remainder had various physical illnesses. During the year after the index visit, 23 (25%) of the patients died and another 14 (15%) deteriorated clinically. Physical causes of weight loss were clinically evident on the initial evaluation in 55 of 59 patients. The four patients in whom the diagnosis was initially missed had cancer, and in only one of these patients was the illness truly occult. Because diagnoses were usually made rapidly in patients with a physical cause of weight loss, we conclude that involuntary weight loss is rarely due to "occult" disease. We developed a decision rule that used six attributes to correctly identify 57 of 59 patients (97%) with a physical cause of weight loss and 23 of 32 patients without. Thus, our rule may help in the early triage of patients with involuntary weight loss.