Although there is some evidence of an association between loss of a sibling in adulthood and subsequent mortality, there have been no previous studies in which investigators have examined whether the death of a sibling in childhood is associated with adult mortality using total population data. Data on a national cohort born in Sweden in 1973-1982 (n = 717,723) were prospectively collected from the Cause of Death Register until 2013 (i.e., from the ages of 18 years to 31-40 years). Cox proportional hazards models were used to analyze the association between sibling loss during childhood and death in young adulthood. After adjustment for sociodemographic confounders and parental psychosocial covariates, the hazard ratio for all-cause mortality in bereaved siblings versus nonbereaved siblings was 1.39 (95% confidence interval: 1.14, 1.69). Risks were more pronounced for those who lost a noninfant sibling (i.e., >1 year of age) (hazard ratio = 1.53, 95% confidence interval: 1.18, 1.95) and those who lost a sibling in adolescence (i.e., between the ages of 12 and 18 years) (hazard ratio = 1.71, 95% confidence interval: 1.24, 2.35). Excess mortality risk was found for concordant causes of death (i.e., siblings dying from the same causes) but not for discordant causes.