Symptoms of excessive daytime somnolence range from mild to severe. In mild cases, there may be minimal interference with normal daytime function. The hypersomnia can be disabling. When severe the patient finds it difficult to remain awake at times when physically inactive. Excessive daytime somnolence is the chief complaint of the majority of our adult patients. In this paper, we present the findings for 1,000 consecutive patients (755 males and 245 females) who were seen at the Humana Hospital Audubon Sleep Disorders Center. Patients ranged in age from 15 to 83. All patients had a sleep history, medical history and physical, psychological evaluation, polysomnographic evaluation, and other laboratory tests as indicated. Obstructive sleep apnea syndrome was the most prevalent diagnosis for males (84.2%) and females (59.6%). It accounted for over three-fourths of all diagnoses. Hypersomnia secondary to a psychiatric disorder was the next most frequent diagnosis overall (6.1%). A psychiatric disorder was second for females and third for males. Narcolepsy was diagnosed for 5.8% of all patients. This was the second most prevalent diagnosis for males and third for females. Eighteen males (47.4% of all males with a diagnosis of narcolepsy) and 9 females (45.0%) had cataplexy. Nocturnal myoclonus was the primary diagnosis in 2.5% of all patients with excessive daytime somnolence. An additional 49 patients with sleep apnea syndrome and 18 patients with narcolepsy also had periodic leg movements during sleep. A diagnosis of obstructive sleep apnea and narcolepsy was made for 1.3% of patients. The narcolepsy component of this diagnosis was typically made only after the obstructive sleep apnea had been resolved (eg, nasal CPAP, tracheostomy).(ABSTRACT TRUNCATED AT 250 WORDS)