Background: The schedule for affective disorders and schizophrenia for school-age children (K-SADS) is one of the most commonly used standardized diagnostic interviews in child and adolescent psychiatry. Validity studies are scarce, and limited to concurrent validity with other measures and clinical diagnoses.Aims: To evaluate the K-SADS interview in an outpatient child and adolescent psychiatry (CAP) setting with a Longitudinal Expert All Data (LEAD) procedure.Methods: CAP residents performed a K-SADS-PL interview with the revised 2009 version containing the new PDD section on 239 clinically referred outpatients of 6-17 years old and their parent(s). A consensus LEAD diagnosis by two senior clinicians 1.2 (SD=0.6) years later was based on clinical records including the K-SADS and subsequent information from further assessments, information from teachers and other informants, outcome of treatment, and at least three visits after the K-SADS.Results: Predictive validity for K-SADS vs LEAD diagnoses were good-to-excellent for broader categories of anxiety disorders (=0.94), depressive (=0.91), behavioural (=0.91) and tic (=0.81) disorders, good for ADHD (=0.80), and good-to-moderate for autism spectrum disorders (=0.62). Bipolar, psychotic, and eating disorders were too few to be analysed.Conclusion: The K-SADS diagnoses elicited from an interview with the child and one from parents on one occasion have an excellent validity for most major child psychiatric disorders. ADHD can be reliably diagnosed at one visit, but clinicians need to stay alert for possible undiagnosed ADHD. Diagnosing autism with K-SADS-PL 2009 version at one visit is not advisable.