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High premorbid IQ test scores are related to a reduced rate of later total mortality, although little is known about the shape of this association (ie, dose-response versus threshold), or the role of mediating and confounding factors in explaining it. Additionally, the link between IQ and cause-specific mortality has been little explored.A cohort of over 1 million Swedish men who underwent IQ testing at military service conscription at about 18 years of age was followed for mortality experience until middle age.An average of 20 years of follow-up gave rise to 14,498 deaths in an analytical sample of 994,262. In basic analyses adjusting for age, year of birth, and conscription testing center, lower IQ scores were associated with an elevated risk of all-cause mortality (HRper 1-SD decrease in IQ; 1.32; 95% confidence interval = 1.30–1.34). This relation was incremental across the full IQ range, and was robust to adjustment for indicators of childhood social circumstances. The association did not appear to be mediated by factors measured concurrent with IQ (blood pressure, body mass index, or cigarette smoking), nor was it attributable to reverse causality. However, controlling for education (a close correlate of IQ) led to marked attenuation. IQ was also associated with mortality from accidents, coronary heart disease, and suicides, but not cancer.In this large cohort we found a robust stepwise relation between early adult IQ and risk of total and accident mortality in men.