The cycle of rapid eye movement sleep is associated with penile erection. The study of sleep-related erections (SRE) contributes to the differential diagnosis for erectile dysfunction (ED), which can be due to an organic or a psychogenic cause. The pioneeristic de- vices for the study of SRE could only evaluate penile tumescence; nowadays also radial and axial penile rigidity can be evaluated. However only radial rigidity can be monitored continuously even if axial rigidity is more relevant in the study of ED. The study of SRE does not provide a neat differential diagnosis between organic and psychogenic ED, but this diagnostic tool has an important role if the data which can be obtained are interpreted within a wider study of the patient.