BACKGROUND: Exposure to adverse childhood experiences (ACE) have been found to have profound negative consequences on an individuals' health. Non-suicidal self-injury (NSSI) is a clinically complex and serious global health issue and is closely related to suicide attempts. Previous research has found associations between ACE and NSSI and suicide attempts in clinical samples. However, this association has to our knowledge not been studied to this extent in a sample of forensic psychiatric patients. The aim of this study was therefore to describe the prevalence of adverse childhood experiences (ACE) and their associations with non-suicidal self-injury (NSSI) and/or suicide attempts in forensic psychiatric patients. METHODS: The current study is a cross-sectional study of a consecutive cohort of 98 forensic psychiatric patients (86.7% male) in Sweden. We invited 184 patients with a predicted stay of > 8 weeks who had been cleared for participation by their treating psychiatrist. Of these, 83 declined and 98 eligible patients provided informed consent. Information on ACE, NSSI, and suicide attempts derived from files, self-reports (Childhood Trauma Questionnaire-Short Form; CTQ-SF), and interviews were compared separately among participants with and without NSSI or suicide attempts using t-tests. The dose-response association between ACE and NSSI/suicide attempts was analysed using binary logistic regression. RESULTS: In file reviews, 57.2% of participants reported physical abuse, 20% sexual abuse, and 43% repeated bullying by peers during childhood. NSSI and suicide attempts were associated significantly with CTQ-SF total scores, with medium effect sizes (d = .60 to .63, p < .01), and strongly with several CTQ-SF subscales. Parental substance abuse was also associated with NSSI (p = .006, OR = 3.23; 95% confidence interval [CI] = 1.36 to 7.66) and suicide attempts (p = .018, OR = 2.75; 95% CI = 1.18 to 6.42). Each additional ACE factor predicted an increased probability of NSSI (p = .016, OR = 1.29; CI = 1.04 to 1.59) but not of suicide attempts. When anxiety and depressive disorders were included in the model, ACE remained a significant predictor of NSSI. CONCLUSIONS: We report extensive ACE, from both files and self-reports. When comparing groups, correlations were found between ACE and NSSI, and ACE and suicide attempts among forensic psychiatric patients. ACE seem to predict NSSI but not suicide attempts in this group, even when controlling for affective and anxiety disorders. Early ACE among forensic psychiatric patients, especially physical and emotional abuse and parental substance abuse, have important impacts on self-harming behaviours that must be acknowledged both by the institutions that meet them as children and in their later assessment and treatment.