Among 1195 twins born in the Collaborative Perinatal Project, for whom information was available, 219 (18.33%) were found to have malformations, 179 (14.98%) single and 40 (3.35%,) multiple. The frequency of malformations among twins was significantly higher than that among singletons from the same population, but the difference was entirely contributed by MZ twins. This holds true for both major and minor malformations. The frequency among Negro twins was higher than among white, and among male twins higher than among female. Twins had more malformations of the central nervous, musculoskeletal, ear, respiratory, cardiovascular, and alimentary systems and fewer malformations of the genitourinary and integumentary systems than singletons. In a significant number of cases when one twin of a pair had a malformation, the cotwin also had a malformation though not necessarily the same as that of the first twin.