The Schimmelpenning-Feuerstein-Mims syndrome (SFM), characterized by linear nevus sebaceous and ocular and neurologic abnormalities, is a sporadic condition without known familial cases or etiology. We report the occurrence of SFM in only one of two monozygotic (MZ) twins. After considering a variety of possible causative mechanisms, we suggest that a postzygotic dominant lethal mutation in mosaic form may best explain SFM and the discordancy for SFM in these MZ twins.