AbstractAsymptomatic testicular enlargement is common in mentally retarded males. This enlargement includes males with non‐specific forms of mental retardation as well as a subgroup of males who have a chromosomally recognizable feature involving the X‐chromosome (“fragile X” chromosome). We examined the testes of a group of 25 mentally retarded males with macro‐orchidism including both fragile X and nonfragile X patients. Apart from large size, the testes scanned were all normal in appearance. In particular, there was no evidence for neoplasia. There were no distinguishing features between the fragile X and the non‐fragi