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Lower intelligence is a risk factor for several specific mental disorders. It is unclear whether it is a risk factor for all mental disorders, and whether it might be associated with illness severity. We examined the relation of premorbid intelligence with risk of hospital admission and with total admission rates, for the whole range of mental disorders.Participants were 1,049,663 Swedish men who took tests of intelligence on conscription into military service and were followed up with regard to hospital admissions for mental disorder, for a mean of 22.6 years. International Classification of Diseases diagnoses were recorded at discharge from the hospital.Risk of hospital admission for all categories of mental disorder rose with each point decrease in the 9-point IQ score. For a standard deviation decrease in IQ, age-adjusted hazard ratios (95% confidence interval) were 1.60 for schizophrenia (1.55–1.65), 1.49 for other nonaffective psychoses (1.45–1.53), 1.50 for mood disorders (1.47–1.51), 1.51 for neurotic disorders (1.48–1.54), 1.60 for adjustment disorders (1.56–1.64), 1.75 for personality disorders (1.70–1.80), 1.75 for alcohol-related (1.73–1.77), and 1.85 for other substance-use disorders (1.82–1.88). Lower intelligence was also associated with greater comorbidity. Associations changed little on adjustment for potential confounders. Men with lower intelligence had higher total admission rates for mental disorders, a possible marker of clinical severity.Lower intelligence is a risk factor for the whole range of mental disorders and for illness severity.