An X-linked pattern of transmission observed in four families with familial mental retardation in several generations was associated with a probable secondary constriction at the distal end of the q arms of the X chromosome. Twenty retarded males and no retarded females were observed. All available live retarded males and most of their normal mothers were found to have the abnormal X chromosome. The marker chromosome was shown to be the X chromosome in each case by Giemsa banding. In affected male and female carriers the marker chromosome varied in appearance and was not present in all metaphases. The significance of this study in relation to previously reported pedigrees showing non-specific X-linked mental retardation is discussed.