A review of the sleep of 31 patients 45 years old or older undergoing nocturnal penile tumescence studies showed that 19 had a previously undiagnosed sleep disorder. Of the patients 9 had periodic leg movements in sleep, 9 had sleep apnea and 1 had both disorders. In 10 of these patients the sleep disorders affected nocturnal penile tumescence by disrupting sleep and causing brief periods of detumescence, movement artifacts and delays in the tumescing phase of nocturnal penile tumescence. These disruptions resulted in an apparently abnormal nocturnal penile tumescence that appeared as if the patient had difficulty in achieving or maintaining an erection. The nocturnal penile tumescence disruptions may have reflected only a disruption of the necessary conditions for normal nocturnal penile tumescence to occur, namely adequate sleep and rapid eye movement sleep. The results strongly suggest that failure to measure concurrent sleep parameters and screen for occult sleep disorders could result in the incorrect diagnosis of abnormal nocturnal penile tumescence.