Epithelioma adenoides cysticum (EAC) is a well-known genodermatosis which follows an autosomal dominant mode of inheritance. Questions still remain, however, concerning an apparent excess of females with EAC. An analysis of nineteen previously published pedigrees and one newly ascertained pedigree, which in the aggregate included over 175 cases of EAC, induced no excess of affected females, but rather, a marked deficit males. This deficiency was not the consequence of sex linkage or decreased viability, and was most evident in large pedigrees and sibships, sibships not including the probands, and late birth orders. In these situations, the penetrance of EAC in male gene carriers was only 50%, but was close to 100% in female gene carries. Under maximum detection conditions, i.e. small pedigrees, small sibships, sibships containing the proband, and early birth orders, the penetrance in males increased to 85% and was again close to 100% in females. These findings suggested that the deficit of affected males was best explained by a comination of lessened expressivity and penetrance, effects which were magnified under situations of poor detection. A review of familial cases of EAC indicated no consistent associated anomalies such as those that characterize the naevoid basal cell carcinoma syndrome.