The impact of family factors, particularly the sibling status, on the development and course of eating disorders has been rarely investigated.Therefore, the aimof the present study was to assess a putative association between sibling status and self-rated family dysfunctionality in a large sample of study participants with bulimia nervosa and anorexia nervosa. A total cohort of n = 568 outpatients aged twelve years and older, of whomhalf had the diagnosis of anorexia (n = 288, 50.7 %), was assessed for self-rated family dysfunctionality using the well-validated German General Family Questionnaire (FB-A), while the symptom level (Global Severity Index) was determined using the SCL-90-Rsymptomchecklist. Patientswith anorexiaweremore frequently only children compared to the bulimia group who had generallymore siblings (22.6%vs. 14.4%, p = 0.012). In patients with bulimia nervosa, bivariate analyses revealed a significant positive association between the presence of siblings and the degree of family dysfunctionality (only child: 33.6 ± 17.6, sibling child: 39.3 ± 15.5, p = 0.043). Linear regressionmodels adjusted for age, body-mass index, parents living in separation, and the Global Severity Index (GSI) confirmed a significant association between higher family dysfunctionality as a dependent variable and sibling status (beta = 0.163, 95%confidence interval [CI] = 1.431; 12.774, p = 0.014).No such relationship was found in the anorexia group (p = 0.418).The differential associations between sibling status and self-rated family dysfunctionality found for the two eating-disorder entities may indicate differences in the pathogenesis of anorexia and bulimia nervosa.