Poor family relationships during childhood have been shown to have long-term negative effects on an offspring's health. However, few studies have followed the offspring to retirement age, and relatedly, knowledge about the link between poor family relationships and premature death is scarce. The aim of this study was to examine the association between poor family relationships in adolescence and the risk of premature death, even when considering other adverse childhood conditions. Prospective data from the Stockholm Birth Cohort study were used, with 2636 individuals born in 1953 who were followed up until age 65. Information on family relations was based on interviews with the participants' mothers in 1968. Information on mortality was retrieved from administrative register data from 1969-2018. Cox proportional hazards regressions showed that poor family relationships in adolescence were associated with an increased risk of premature death, even when adjusting for childhood conditions in terms of household social class, household economic poverty, contact with the child services, parental alcohol abuse, and parental mental illness (Hazard Ratio (HR), 2.08, 95% Confidence Interval (CI), 1.40-3.09). The findings show that poor family relationships in adolescence can have severe and long-lasting health consequences, highlighting the importance of early interventions.