OBJECTIVE: Delayed sleep phase syndrome is a common but little reported cause of severe insomnia. Since it was first described, few detailed reports of delayed sleep phase syndrome have appeared, and treatment methods have not been reviewed. From the literature, the authors provide diagnostic descriptions and review treatment methods, and from their sleep disorder clinic, they describe the management and outcome of the largest series of patients with delayed sleep phase syndrome thus far reported. METHOD: The authors reviewed all articles with primary data on delayed sleep phase syndrome published through 1993 and add data from a group of 33 patients at their sleep disorder clinic. RESULTS: Delayed sleep phase syndrome involves undesirably late bedtimes and arising times, early night insomnia, and poor morning alertness but lack of insomnia on vacations. The mean bedtime and arising time for the 33 patients were 4:00 a.m. and 10:38 a.m., respectively. Twenty-five patients were, or had been, depressed. Individual responses to treatments varied widely. Seventeen patients showed little treatment response. Delayed sleep phase syndrome had a worse treatment outcome than other sleep disorders. CONCLUSIONS: Delayed sleep phase syndrome presents in a heterogeneous manner. In the sleep disorder clinic population, it was often associated with major depression and was more resistant to treatment than other sleep disorders. Multiple and varied treatments are required.