OBJECTIVE: The authors seek to clarify, from both an epidemiologic and genetic perspective, the major risk factors for bulimia nervosa and to understand the relationship between narrowly defined bulimia and bulimia-like syndromes. METHOD: Personal structured psychiatric interviews were conducted with 2,163 female twins from a population- based register. Psychiatric disorders were assessed using DSM-III-R criteria. RESULTS: Lifetime prevalence and risk for narrowly defined bulimia were 2.8% and 4.2%, respectively. Including bulimia-like syndromes increased these estimates to 5.7% and 8.0%, respectively. Risk factors for bulimia included 1) birth after 1960, 2) low paternal care, 3) a history of wide weight fluctuation, dieting, or frequent exercise, 4) a slim ideal body image, 5) low self-esteem, 6) an external locus of control, and 7) high levels of neuroticism. Significant comorbidity was found between bulimia and anorexia nervosa, alcoholism, panic disorder, generalized anxiety disorder, phobia, and major depression. Proband-wise concordance for narrowly defined bulimia was 22.9% in monozygotic and 8.7% in dizygotic twins. The best-fitting model indicated that familial aggregation was due solely to genetic factors with a heritability of liability of 55%. A multiple threshold model indicated that narrowly defined bulimia nervosa and bulimia-like syndromes represented different levels of severity on the same continuum of liability. CONCLUSIONS: The liability to fully syndromal bulimia nervosa, which affects around one in 25 women at some point in their lives, is substantially influenced by both epidemiologic and genetic risk factors. The same factors that influence the risk for narrowly defined bulimia also influence the risk for less severe bulimia-like syndromes.Abstract Teaser