We conducted a study to investigate diagnostic accuracy of NPTM (Nocturnal Penile Temescence Test) and EST (Erotc Stimulation Test) in differential diagnosis of impotence, and to elucidate whether both of two tests are better than doing one of those tests and how to arrange those in the steps of diagnostic process. Full diagnostic work-up including polysomnographic NPTM and EST was performed in 45 impotents. Based on the etiology of impotence in detail proved by other sophisticated diagmostic methods, doagnostic accuracy of those tests were evaluated. Overall accuravy rate of NPTM proved to be 82%;84% (21 out of 25 patients) in diagnosing psychogenic impotence and 80% (16 out of 18 patients) in diagnosing organic impotence. And EST without considering the degree of patients's sexual drive to erotic stimulation showed overall accuracy rate of 72%;68% (17 out of 22 patients) in diagnosing psychogenic impotence and 75% (15 out 23 patients) in diagnosing organic impotence. However, EST showed better results with overall accuracy rate of 80% in 29 patients with Grade II or III (moderate or good degree) sexual drive; 88%(16 out of 18 patients) in diagnosing psychogenic impotence and 72% (8 out of 11 patients) in diagnosing organic impotence. These results suggest that it is desirable to perform EST as the initial step in diagnostic work-up for impotent patients and that test provides high diagnostic reliability when the outcome proves to be psychogenic impotence under the sexual drive with moderate or good degree. And NPTM can be preserved for the cases when results except above one are encountered following EST.