Background: Knowledge about the presentation of psychopathology in preschool age and associated risk factors is fundamental to preventive intervention before schooling. Aims: To investigate the full spectrum of psychiatric diagnoses in general population children at the period of transition from preschool to school. Methods: A sample of 1585 children from the Copenhagen Child Cohort, CCC2000 aged 5-7 years was assessed using the Development and Well-Being Assessment (DAWBA) with diagnostic classification by experienced clinicians. Perinatal, sociodemographic and socio-economic data was obtained from Danish national registries. Results: The prevalence of any ICD-10 psychiatric disorder was 5.7% (95%CI: 4.4-7.1). Pervasive developmental disorders (PDD) were found in 1.3% (95%CI: 0.8-1.8) and behavioural and hyperkinetic disorders were found in 1.5% (95%CI: 0.9-2.1) and 1.0% (95%CI: 0.4-1.6), respectively. Emotional disorders were found in 2.9% (95%CI: 1.9-40). More boys were diagnosed with PDD, behavioural disorders and tics. No gender differences were found in hyperactivity disorders (HD) and emotional disorders. Co-morbidity was frequent, in particular between HD and PDD, but also between HD and emotional disorder and behavioural disorder. Teenage mothers, single parents and low household income the first two years after the child’s birth were associated with a three-to fourfold increased risk of psychiatric disorder in the child at age 5-7 years. Conclusion: The study results point to two ‘windows of opportunity’ for prevention. In the earliest postnatal years, prevention should target families at socio-economic risk; and in the years before schooling, intervention should focus on children with symptoms of PDD, HD, and behavioural disorders.