Summary The present study investigated the sympathetic/parasympathetic balance during non-rapid eye movement (NREM) and rapid eye movement (REM) sleep in patients with psychogenic and organic erectile dysfunction. The cardiac autonomic balance was assessed from the power of the low frequency (LF) and high frequency (HF) spectral components of heart-rate variability in 11 patients with psychogenic erectile dysfunction and 11 patients with organic erectile dysfunction as determined by monitoring sleep-related erections. Spectral analysis of heart-rate variability was calculated for at least four successive 4-min epochs of electrocardiogram recordings during NREM sleep and for all available 4-min epochs during REM sleep. Statistical analysis revealed that organic patients had a significantly higher LF/HF ratio (P< 0.01) during both stages of sleep, which resulted from a significantly lower power in the HF component (P< 0.004) and higher power in the LF component (P< 0.01) in these patients, in both REM and NREM sleep stages. These results demonstrate that patients complaining of daytime sexual dysfunction and found by sleep-related erection monitoring to suffer from organic erectile dysfunction, have altered cardiac autonomic balance during both stages of sleep.