A series of 235 consecutive patients refferred to the Stanford University Sleep Disorders Clinic with the complaint of excessive daytime sleepiness (EDS) were investigated extensively. A satisfactory final diagnosis involving a consistent syndrome or pathogenic process was made in all but 7 patients. In the course of this work a variety of tests, including prolonged polygraphic monitoring of multiple variables and CSF measurements before and after probenecid ingestion, were utilized. Different syndromes were confirmed (harmonious hypersomnia, subwakefulness syndrome); the definitions of others were clarified and extended (narcolepsy, drug dependency, periodic hypersomnia associated with menstruation, upper airway sleep apnea in children). Two new entities were tentatively identified (narcolepsy with sleep apnea, the neutral state syndrome). Narcolepsy and upper airway sleep apnea accounted for the majority of the cases (199). A strategic schema utilizing specific categories and frequency of occurrence in the case series is presented to improve the diagnosis of the complaint of excessive daytime sleepiness by the practicing physician. This case series was analysed in order to develop tentatively a meaningful nosology.