We report on a family with X-linked hydrocephalus: progressive increase in head circumference (OFC) led to the diagnosis in 3 patients; however, in 5 with normal OFC, the initial diagnosis had been "nonspecific" mental retardation, until identification of relatedness between 3 macrocephalic boys suggested segregation of a major Mendelian gene. Moderate to severe hydrocephaly was present in all macrocephalic patients and in 3 of the 5 with normal OFC, but CT-scan of the brain did not show aqueductal stenosis in any of them. We stress the importance of a brain CT-scan in every male with "nonspecific" mental retardation, especially in cases with X-linked inheritance, because confirmation of X-linkage has important genetic counseling consequences.