Objective To evaluate the association between adverse childhood experiences – e.g. abuse, neglect, domestic violence and parental separation, substance use, mental illness or incarceration – and the health of young adults in eight eastern European countries. Methods Between 2010 and 2013, adverse childhood experience surveys were undertaken in Albania, Latvia, Lithuania, Montenegro, Romania, the Russian Federation, The former Yugoslav Republic of Macedonia and Turkey. There were 10 696 respondents – 59.7% female – aged 18–25 years. Multivariate modelling was used to investigate the relationships between adverse childhood experiences and healthharming behaviours in early adulthood including substance use, physical inactivity and attempted suicide. Findings Over half of the respondents reported at least one adverse childhood experience. Having one adverse childhood experience increased the probability of having other adverse childhood experiences. The number of adverse childhood experiences was positively correlated with subsequent reports of health-harming behaviours. Compared with those who reported no adverse experiences, respondents who reported at least four adverse childhood experiences were at significantly increased risk of many health-harming behaviours, with odds ratios varying from 1.68 (95% confidence interval, CI: 1.32–2.15) – for physical inactivity – to 48.53 (95% CI: 31.98–76.65) – for attempted suicide. Modelling indicated that prevention of adverse childhood experiences would substantially reduce the occurrence of many healthharming behaviours within the study population. Conclusion Our results indicate that individuals who do not develop health-harming behaviours are more likely to have experienced safe, nurturing childhoods. Evidence-based programmes to improve parenting and support child development need large-scale deployment in eastern European