To determine the prevalence and psychological features of fragile X syndrome among Japanese, cytogenetic and psychological studies were conducted on 183 institutionalized male retardates. The induction of fra (X) (q27) by MEM-FA medium or fluorodeoxyuridine treatment revealed that 6 (3.3%) patients had fragile X syndrome. Psychological tests in the 6 patients with fragile X syndrome [Fra (X)], 6 control subjects with autism [AU] and 6 with simple mental retardation [MR] showed the following results : (1) on SM social maturity test, overall scores of social skills in Fra (X) were between those in AU and those patients in MR. Behavioral autonomy was much better in Fra (X) than in AU patients ; (2) on CLAC-Ⅱ behavior evaluation, Fra (X) patients had a tendency toward lower developmental levels in verbal and nonverbal expression as well as in handling compared to AU patients ; (3) on DSM-Ⅲ-R test, 3 of the 6 Fra (X) males met the criteria for autism. Fra (X) patients had fewer failures in recognizing others' existence or emotions, to develop relationships with others and to use language appropriately compared to AU patients, but had more defects in imitation of others and adaptation to environmental changes than MR patients. The similarity in the prevalence of the fragile X syndrome in the Japanese male retardates to that reported in Caucasians suggests that Fra (X) is a common genetic disorder affecting all ethnic populations. From the psychological findings, we concluded that Fra (X) patients have strong social aversion and defects in verbal and nonverbal expression which sometimes lead to autism.