The clinical and cytogenetic features of 15 families with mental retardation linked to the fragile site on the X chromosome are presented. The 15 propositi were all prepubertal, and one was a girl. Although the clinical picture varied in severity, it was sufficiently constant to suggest the diagnosis from the facial features and the encephalopathy with language retardation and disturbed behavior. Macroorchidism was not seen before puberty. The fragile X chromosome was found in seven of the nine mothers studied and in two mildly retarded sisters and has also been demonstrated in fibroblasts in eleven subjects with the abnormality.