Determination of plasma testosterone in 105 patients with Klinefelter's syndrome 16--45 years of age revealed that at each 5-year age interval values of male hormone were lower than in a control group of 25 normal adolescents and 85 fertile and potent men. Analysis of heterosexual development by means of the HTDM Questionnaire in 110 patients with Klinefelter's syndrome age 21--40 years, who were examined mainly for sterility and signs of imperfect somatosexual development, revealed a distinct retardation of sociosexual development compared to that of 325 normozoospermic men from sterile marriages. For eight out of 12 items on the HTDM Questionnaire the differences were statistically significant. Examination by means of the SAM Questionnaire in these two groups revealed that the sexual activity of examinees with Klinefelter's syndrome was significantly weaker than that of fertile and potent men. For 15 out of 18 items on the SAM Questionnaire the differences were statistically significant.