Background: Many disorders in childhood and adolescence were already present in the preschool years. However, there is little empirical research on the prevalence of psychiatric disorders in young children. A true community study using structured diagnostic tools has yet to be published. Methods: All children born in 2003 or 2004 in the city of Trondheim, Norway, who attended the regular community health check-up for 4-year-olds (97.2% of eligible children) whose parents consented to take part in the study (N = 2,475, 82.0%) were screened for behavioral and emotional problems with the Strengths and Difficulties Questionnaire (SDQ). A screen-stratified subsample of 1,250 children took part in a furthermore comprehensive study including a structured diagnostic interview (the Preschool Age Psychiatric Assessment, PAPA), which 995 parents (79.6%) completed. Results: The estimated population rate for any psychiatric disorder (excluding encopresis - 6.4%) was 7.1%. The most common disorders were attention deficit hyperactivity disorder (1.9%), oppositional defiant disorder (1.8%), conduct disorder (0.7%), anxiety disorders (1.5%), and depressive disorders (2.0%). Comorbidity among disorders was common. More emotional and behavioral disorders were seen in children whose parents did not live together and in those of low socioeconomic status. Boys more often had attention-deficit/hyperactivity disorder (ADHD) and depressive disorders than girls. Conclusions: The prevalence of disorders among preschoolers was lower than in previous studies from the USA. Comorbidity was frequent and there was a male preponderance in ADHD and depression at this early age. These results underscore the fact that the most common disorders of childhood can already be diagnosed in preschoolers. However, rates of disorder in Norway may be lower than in the USA.