BACKGROUND: Longitudinal studies indicate that future schizophrenia patients exhibit lower IQ than healthy controls. Recent studies suggest that future patients with other mental illnesses obtain lower pre-morbid IQ. The aims of this study were to compare pre-morbid IQ among five diagnostic categories and normal controls, to examine the distribution of pre-morbid IQ, and to investigate the relationship between pre-morbid IQ and risk of mental illness.MethodA total of 7486 individuals hospitalized with psychiatric disease and 20 531 controls. IQ was measured at the draft board and hospital diagnoses [schizophrenia (Sz), non-schizophrenic, non-affective psychoses (NSAP), affective (AD), personality (PD) and neurotic/stress disorders (ND)] were followed up to ages 43-54 years. Individuals hospitalized 1 year after appearing before the draft board were excluded. RESULTS: All future patients obtained significantly lower pre-morbid IQ than controls (3-7 IQ points), AD had the highest IQ and PD the lowest. In each diagnostic category, decreasing IQ was associated with an increasing risk of becoming a patient [odds ratios (ORs) 0.5-2.5 over the full IQ spectrum]. IQ distributions was nearly normal and uni-modal. CONCLUSIONS: IQ deficits in each diagnostic category may reflect different functional patterns and temporal vicissitudes of the specific pathogenetic processes involved in different mental disorders.