This study examined the nature and prevalence of diagnostically defined sleep disorders, including Sleep Onset Insomnia (SOI) and Night Waking Insomnia (NWI), in a sample of 183 young children admitted to an early childhood psychiatric day treatment program. A semi-structured diagnostic interview, the Diagnostic Infant and Preschool Assessment, was used to assess for sleep and other psychiatric disorders. Daily sleep diaries and the Child Behavior Checklist were also examined. 41 % of children met criteria for a sleep disorder; 23 % met diagnostic criteria for SOI and 4 % met criteria for NWI, with an additional 14 % meeting criteria for both (SOI + NWI). Sleep-disordered children demonstrated longer latency to sleep onset, longer and more frequent night awakenings, less total sleep, and lower sleep efficiency than non-sleep disordered participants. Diagnosable sleep disorders, particularly SOI, were quite common in this acute clinical sample, exceeding previous estimates obtained in community and pediatric practice samples.