BACKGROUND: Most research investigating the relationship between IQ and risk of mental disorder has focused on schizophrenia. AIMS: To illuminate the relationship between IQ test scores in early adulthood and various mental disorders. METHOD: For 3289 men from the Copenhagen Perinatal Cohort, military IQ test scores and information on psychiatric hospitalisation were available. We identified 350 men in the Danish Psychiatric Central Register, and compared the mean IQ test scores of nine diagnostic categories with the mean scores of 2939 unregistered cohort controls. RESULTS: Schizophrenia and related disorders, other psychotic disorders, adjustment, personality, alcohol and substance-use-related disorders were significantly associated with low IQ scores, but this association remained significant for the four non-psychotic disorders only when adjusting for comorbid diagnoses. For most diagnostic categories, test scores were positively associated with the length of the interval between testing and first admission. ICD mood disorders as well as neuroses and related disorders were not significantly associated with low IQ scores. CONCLUSIONS: Low IQ may be a consequence of mental disease or a causal factor in psychotic and non-psychotic disorders.