Nine men with X-linked mental retardation (MR) belonging to the family originally described by Renpenning and his colleagues have been reexamined and restudied. None of these men have the fragile X chromosome, and none have macro-orchidism; their testes range in size from small to average. All but one are severely retarded; one has an IQ of 70, the mean IQ of the remainder is 30 with a range of 18 to 45. They are short in stature, have small or normal-size ears and lower jaws, and a moderate degree of microcephaly, the mean skull circumference being just below two standard deviations from the normal.