In the course of several ongoing studies, the authors have encountered men and women who display a form of body dysmorphic disorder in which they become pathologically preoccupied with their degree of muscularity. This condition, which the authors have tentatively termed "muscle dysmorphia," may cause severe subjective distress, impaired social and occupational functioning, and abuse of anabolic steroids and other substances. Epidemiologic data suggest that muscle dysmorphia, though rarely recognized, may afflict substantial numbers of Americans. The authors summarize the features of muscle dysmorphia, present several case examples, and offer proposed diagnostic criteria that may be useful for subsequent research.