BACKGROUND: Few studies have examined the correlation between social circumstances during childhood and adult suicide behaviour in a prospective design. How indicators of impaired childhood development affect the risk of suicide behaviour before the age of 50 years are analysed in this study. METHOD: 9359 Men born in Copenhagen, Denmark, in 1953, who had completed conscription medical examination at approximately 18 years of age and for whom birth certificates had been traced were followed from 1972 to 2003 in Danish health registers regarding suicide or suicide attempts. At the age of 12 years 6856 of these cohort members had completed a cognitive performance test. RESULTS: During follow-up, 92 (1.0%) and 228 (2.4%) of the men, respectively, committed suicide or had one or more suicide attempt. Low body mass index (BMI), low cognitive test score, and mental disorder at the age of 18 years were associated with an increased risk of suicide. After adjustment, mental disorder remained significantly associated with suicide risk (hazard ratio 2.49, 95% CI 1.03 to 6.02), whereas the estimates for cognitive function (HR 0.77, 95% CI 0.59 to 1.01) and BMI (HR 0.81, 95% CI 0.64 to 1.02) were attenuated. In the crude analyses suicide attempt was associated with all indicators, but after adjustment mental disorder (HR 2.64 (95% CI 1.55 to 4.49), no more than a basic school education (HR 2.28, 95% CI 1. 30 to 4.00), BMI (HR 0.87, 95% CI 0.75 to 1.00) and cognitive test score at the age of 18 years (HR 0.85, 95% CI 0.72 to 1.01) were associated with the risk of suicide attempt. CONCLUSION: This study suggests that low BMI, low cognitive function, and mental disorder in early adulthood are markers of an increased risk of suicide among men born in Denmark in 1953.