The diagnosis of narcolepsy can be problematic. Most sleep laboratories use polygraphic testing to establish the diagnosis. One polygraphic recording followed by a single multiple sleep latency test (MSLT) is used to differentiate the causes of syndromes with complaints of daytime somnolence. Prospective investigations have demonstrated that patients with periodic leg movements or upper airway resistance syndrome may present abnormal sleep latencies and more than one sleep onset rapid eye movement period (SOREMP) during MSLT. On the other hand, investigations of patients with daytime sleepiness and cataplexy have shown that the MSLT may not show more than one SOREMP. The combination of history of cataplexy and more than one SOREMP during MSLT is the best clinical determinant of narcolepsy. History of daytime sleepiness and presence of more than one SOREMP during MSLT, however, is a poorer discriminant of narcolepsy than history of cataplexy, particularly in an aging population.