Comparison of the incidence of enuresis in monozygotic and same-sex dizygotic twins revealed that bed-wetting after the fourth birthday was present in 146 out of 676 twin children, an incidence of 22%. Monozygotic twins are concordant for enuresis about twice as frequently as dizygotic twins, a statistically significant difference. This finding together with the family studies is consistent with the view that the basis for enuresis is principally genetic. The incidence of nail-biting in children with enuresis is about the same as in the non-enuretic. Finger-sucking after 3 years of age is more frequent in children with enuresis. No relation was found between the incidence of enuresis and the ABO blood types.