In a group of 47 cases of true Klinefelter''s syndrome"mainly examined for sterility, attention has been paid to their educational standard, school results, occupation, and Raven''s test results of their mental level. None of the 47 subjects with female nuclear sex attained university-standard education. The school results were poor in the case of 20 of them, and excellent only in three. Subnormal results of Raven''s test (below 80) were found in 12 out of the 47 subjects examined. Only in the case of two patients was the result of the test above average (more than 120). The average value of Raven''s test for the whole group was 91.9 (s = 16.4). These findings are explained by the simultaneous disorder of the sex glands as well as of the nervous system of these subjects due to a developmental disturbance which has taken place before birth.