IMPORTANCE Nonsuicidal self-injury (NSSI) and suicidal self-injury are very harmful behaviors and are associated with several psychiatric disorders. In the recently developed fifth edition of the DSM, NSSI and suicidal behavior disorder are for the first time introduced as conditions in their own right instead of symptoms of other psychiatric disorders. It is unclear to what extent NSSI and suicidal self-injury share the same underlying biological mechanisms and are influenced by the same environmental factors. OBJECTIVE To determine the relative importance of genetic and environmental influences on the variation in NSSI and suicidal ideation and their covariation. DESIGN, SETTING, AND PARTICIPANTS Classical twin design using a sample of 10 678 male and female adult twins (mean [SD] age, 32.76 [6.99] years) from the Australian Twin Registry, a population-based twin registry. Between 1996 and 2009, the twins participated in semistructured telephone interviews that primarily focused on psychiatric disorders. MAIN OUTCOMES AND MEASURES Lifetime presence of self-reported NSSI and suicidal ideation. RESULTS The prevalences of NSSI and suicidal ideation were 4.7%and 26.5%, respectively, and individuals who engaged in self-harm were much more likely to report suicidal ideation (odds ratio = 8.39; 95%CI, 6.84-10.29). Results from a bivariate genetic model indicated that genetic factors explain a substantial part of the variance in both NSSI (37%for men and 59% for women) and suicidal ideation (41% for men and 55%for women), while residual influences (including nonshared environmental influences and measurement error) explain the remainder of the variance. Shared (family) environment did not seem to play a role. Moreover, both behaviors were strongly correlated (r = 0.49 for men and 0.61 for women), and this correlation was largely explained by overlapping genetic influences (76%for men and 62%for women), whereas residual influences accounted for the remainder of the phenotypic correlation. CONCLUSIONS AND RELEVANCE Results indicated that the substantial correlation between NSSI and suicidal ideation is largely driven by overlapping genetic factors, suggesting that the 2 behaviors share similar biological underpinnings. Overlapping residual influences also explain part of the covariance between the 2 traits. Future research should further investigate which genetic and environmental influences underlie the vulnerability to NSSI and suicidal ideation.