The descriptive epidemiology of conjoined twinning in the United States was investigated using data from the Birth Defects Monitoring Program (BDMP), a nationwide congenital malformations surveillance system that monitors discharge diagnoses associated with a third of the births in the United States. This is the largest recorded series concerning conjoined twins; data were analyzed on 7,903,000 births monitored by the BDMP in the period 1970-1977. The analysis identified 81 sets of conjoined twins, for a crude incidence of 10.25 per million births. The most common types of conjoined twins were thoracoomphalopagus (28%), thoracopagus (18%), omphalopagus (10%), parasitic twins (10%), and craniopagus (6%). Conjoined twins are more common among females than males, and in nonwhites than whites. No maternal age effect was found. There was no evidence of seasonal or temporal clustering of the cases. The large number of conjoined twins who had birth defects that are not obviously linked to the conjoining (particularly neural tube defects and orofacial clefts) may provide insights into the pathogenesis of birth defects resulting from disordered embryonic migration and fusion.