Kleine-Levin syndrome and periodic hypersomnia are often misdiagnosed initially because there is no objective test for these conditions. To determine the value of the Multiple Sleep Latency Test and polysomnography in this respect, the authors studied four patients with Kleine-Levin syndrome or periodic hypersomnia who had taken the Multiple Sleep Latency Test and undergone polysomnography during the symptomatic episode and/or during the asymptomatic interval. During but not between symptomatic episodes, the Multiple Sleep Latency Test revealed abnormal sleep latencies in all patients, and polysomnography revealed increased rapid eye movement propensity in one patient and a reduction in δ-sleep in two patients. In conclusion, the Multiple Sleep Latency Test and polysomnography are useful in diagnosing Kleine-Levin syndrome and periodic hypersomnia, especially when administered in a standardized fashion during and after the symptomatic period. The authors recommend that polysomnography and the Multiple Sleep Latency Test be performed no earlier than the second night after the onset of a symptomatic episode and the following day to reveal maximal hypersomnolence, and more than 2 weeks after a symptomatic episode to represent the asymptomatic interval.