We report a pair of identical twins with HC with varying extent of left ventricular hypertrophy and some degree of left ventricular outflow tract obstruction. The diagnosis of identical twins was based on the same sex, blood typings, HLA typings and hybridization patterns to four hypervariable DNA probes. Identical twins are derived from a single zygote and are genetically homogeneous human beings. The present report suggests that heterogeneity in the morphologic expression of HC may not be solely attributed to genetic factors. Environmental factors also may play an important role.