We report six males with the syndrome of macroorchidism and mental retardation. The trait is inherited as though X-linked, or possibly autosomal dominant male-limited. We also found no evidence of gonadal dysfunction. Associated abnormalities were abnormal EEG (3/4), seizures (2/6), and one instance each of cervical vertebral fusion, cataract, esophoria, and abnormal cutaneous pigmentation. One woman with a 50% a priori risk of bearing the mutant gene had mental retardation and seizures. Results of Xg blood-group typing were uninformative for the purpose of linkage analysis.