Previous studies have reported strong associations between birth order, maternal age, and suicide, but these results might have been confounded by socioeconomic and other factors. To control for such factors, we compared suicide risk between siblings and studied how maternal age at child birth and birth order influenced risk in a cohort study of 1,690,306 Norwegians born in 1967-1996 who were followed up until 2008. Using stratified Cox regression, we compared suicide risk within families with 2 or more children in which one died from suicide. Altogether, 3,005 suicides occurred over a mean follow-up period of 15 years; 2,458 of these suicides occurred among 6,741 siblings within families of 2 or more siblings. Among siblings, a higher position in the birth order was positively associated with risk; each increase in birth order was associated with a 46% (adjusted hazard ratio = 1.46, 95% confidence interval: 1.29, 1.66) higher risk of suicide. For each 10-year increase in maternal age at child birth, the offspring's suicide risk was reduced by 57% (adjusted hazard ratio = 0.43, 95% confidence interval: 0.30, 0.62). Our study suggests that confounding due to familial factors is not likely to explain the associations of birth order and maternal age at child birth with suicide risk.