Background: Previous research has shown that poor family relations in childhood are associated with adverse mental health in adulthood. Yet, few studies have followed the offspring until late adulthood, and very few have had access to register-based data on hospitalisation due to psychiatric illness. The aim of this study was to examine the association between poor family relations in adolescence and the likelihood of in-patient psychiatric care across the life course up until age 55. Methods: Data were derived from the Stockholm Birth Cohort study, with information on 2638 individuals born in 1953. Information on family relations was based on interviews with the participants' mothers in 1968. Information on in-patient psychiatric treatment was derived from administrative registers from 1969 to 2008. Binary logistic regression was used. Results: Poor family relations in adolescence were associated with an increased risk of later in-patient treatment for a psychiatric diagnosis, even when adjusting for other adverse conditions in childhood. Further analyses showed that poor family relations in adolescence were a statistically significant predictor of in-patient psychiatric care up until age 36-45, but that the strength of the association attenuated over time. Conclusions: Poor family relationships during upbringing can have serious negative mental-health consequences that persist into mid-adulthood. However, the effect of poor family relations seems to abate with age. The findings point to the importance of effective interventions in families experiencing poor relationships.