Under the organizational aegis of Project Sleep and the Association of Sleep Disorders Centers (ASDC), nearly 5,000 patient records from 11 sleep-wake disorders clinics were analyzed in a cooperative study. These cases represented the diagnostic experience of each of these centers over a two-year period. Each patient underwent polysomnographic study, and his or her condition was diagnosed according to the ASDC classification system, a new, standardized nosology of sleep disorders medicine. The most common major diagnostic category was "disorders of excessive sleepiness (hypersomnia)," 42%; this was followed by "disorders of initiating and maintaining sleep (insomnia)," 26%; "penile tumescence evaluations for impotency," 17%; "parasomnias," 3%; and "disorders of the sleep-wake schedule," 2%. If the impotency evaluations performed in the sleep clinics are removed from the total, leaving only the population that was studied because of sleep complaints, the proportions of the diagnostic categories are hypersomnia, 51%; insomnia, 31%; parasomnias, 15%; and sleep-wake schedule disturbances, 3%. The most prevalent diagnoses in the hypersomnia category were sleep apnea (43%) and narcolepsy (25%). Psychiatric disorders (35%) comprised the most frequent group of insomnia diagnoses, though a variety of other disorders were common. The applications of these results for the practicing physician are discussed.