Objectives: To examine the prevalence and stability of DSM-4-defined sleep disorders from preschool to first grade and to explore the bidirectional relationship between sleep disorders and symptoms of psychiatric disorders. Method: All children born in 2003 or 2004 in Trondheim, Norway, who attended regular community health checkups for 4-year-olds, were invited to participate (97.2% attendance; 82.0% consent rate, n = 2475) in this study. The authors recruited a screen-stratified subsample of 1250 children and interviewed 994 parents (79.6%) using a structured diagnostic interview (the Preschool Age Psychiatric Assessment). Two years later, 795 of the parents completed the interview. Results: There was stability in insomnia (adjusted odds ratio [OR] = 4.03, confidence interval [CI] = 2.83-5.75) and sleepwalking (adjusted OR = 19.28, CI = 4.53-82.10), whereas none of the children with hypersomnia or nightmare disorder at age 4 had the same disorder 2 years later. Insomnia increased the risk for developing symptoms of conduct disorder, major depressive disorder (MDD), and social phobia when the initial levels of insomnia were adjusted for. Symptoms of attention-deficit hyperactivity disorder, oppositional defiant disorder, and MDD at age 4 were statistically linked to insomnia at age 6. Sleepwalking predicted later separation anxiety disorder, whereas hypersomnia was unrelated to symptoms of psychiatric disorders. Conclusion: Insomnia is a prevalent and stable disorder in children and is bidirectionally related to psychiatric symptoms.