Obstetric and neonatal complications have been associated with completed and attempted suicide (suicidal acts) in young offspring. Maternal smoking is one of the most important risk factors for obstetric complications, but the association between prenatal smoking exposure and offspring risk of suicidal acts is unknown. We performed a population-based study of 1,449,333 single births born in Sweden between 1983 and 1996, derived from linked registry data. Maternal smoking and risks of suicidal acts in offspring were estimated using hazard ratios, derived from proportional-hazard models, controlling for potential confounding of parental socio-demographic factors and psychiatric care in first degree relatives. To control for unmeasured familial confounding, a matched case-control analysis of suicidal acts was performed within sibling pairs discordant for prenatal smoking exposure. In the cohort analysis, the adjusted hazard ratio for completed suicide among offspring to women smoking 1-9 cigarettes and at least 10 cigarettes per day were 1.67, 95% confidence interval (CI), 1.29-2.16, and 1.54, 95% CI, 1.12-2.10. For suicidal acts, corresponding hazard ratios were 1.28, 95% CI 1.21-1.35 and 1.48, 95% CI 1.39-1.57, respectively. However, in sibling pairs discordant for suicidal acts and prenatal smoking exposure, we found no evidence that prenatal smoking exposure increased the risk of suicidal acts. We conclude that the association between prenatal smoking exposure and offspring risk of suicidal acts is probably confounded by unmeasured familial factors.