We report on a large 5-generation family with "nonspecific" X-linked mental retardation. Nine living affected males have an IQ between 50 and 70 but have normal stature, facial appearance, and testicular volumes and no other abnormalities. Two obligate carrier females had borderline intellectual abilities and visual-psychomotor difficulties similar to those seen in affected males. Results of chromosome studies, including fragile X, were normal in males and females. Linkage analysis was undertaken, with 19 X-specific chromosomal restriction fragment length polymorphisms (RFLPs), giving a maximal LOD score of 1.60 at a 0.10 recombination fraction for F9, suggesting a localization to distal Xq for the mutant gene in this family.