The authors feel that penile erection during sleep represents the first stage in the investigation of erection problems. The aim is to distinguish between psychogenic impotence the organic impotence. The authors simultaneously perform plethysmography and an electroencephalogram during sleep. Sixty five patients have been investigated in this way. In 44 cases the occurrence of nocturnal penile erection led to the conclusion of a psychogenic origin whilst in 21 cases the results of plethysmography were pathological. Amongst these 21 patients, undergoing vascular investigations, 12 underwent revascularisation of the corpora cavernosa and 12 the insertion of an erection prosthesis. It may be noted that revascularization using the epigastric artery gave widely disparate results: 2 good results amongst 9 patients, with one failure and six inadequate erections. The authors emphasise the fact that there is little alteration in sleep in cases of organic impotence and that phases of paradoxal sleep recur periodically without any great variation in the plethysmographic curve. By contrast, in functional impotence, sleep is invariably disturbed. A whole series of factors are involved in the assessment of the results: total duration of sleep, duration of paradoxal sleep, ratio of paradoxal sleep to total sleep, total erection time, total time in maximum erection and the ratio of erection time to paradoxal sleep. Analysis of sleep is thus a fundamental factor in the proper interpretation of the results of plethysmography in an importance sufferer.