&NA;The study of discordant monozygotic twins may identify important developmental risk factors for adult psychiatric disorder. Differential experiencein uterois one candidate environmental risk factor that may distinguish monozygotic twins. In this report, we examine whether intra‐pair differences in birth weight predicts discordance for adult psychiatric disorders in 527 female monozygotic twin pairs from a population‐based twin registry. Twins were personally interviewed about their lifetime history of DSM‐III‐R alcoholism, anorexia nervosa, bulimia nervosa, generalized anxiety disorder, major depression, panic disorder, social phobia and simple phobia. Birth weight was estimated from birth certificates, or from retrospective maternal, paternal and self‐reports. Conditional logistic regression is used to characterize the association between intra‐pair differences in birth weight and discordance for psychiatric disorder in monozygotic twins. The twin with the heavier birth weight in discordant pairs is (insignificantly) more likely to have a history of alcoholism or bulimia. The twin with the lighter birth weight in discordant pairs is (insignificantly) more likely to have a history of major depression, simple phobia, panic disorder, anorexia nervosa, social phobia or generalized anxiety disorder. For all psychiatric disorders examined, the lighter (or heavier) co‐twin at birth is not systematically the affected twin within discordant pairs.