Probands affected with eating disorders (ED) present a higher number of relatives affected with obsessive-compulsive disorders/tic disorders than a comparison population. Therefore, we hypothesized that ED and obsessive-compulsive disorder (OCD) might share the same biological liability, and that a single major gene might account for that liability. We tested this hypothesis by applying a complex segregation analysis to 141 families of probands affected with ED (89 with anorexia nervosa, restricting and binge-eating types, 52 with bulimia nervosa). Given the hypothesized relationship between OCD and genetic spectrum disorders, we considered these diagnoses as affected phenotype in relatives. In Italian ED families, ED and OCD followed a Mendelian dominant model of transmission. When probands were divided according to co-diagnosis of OCD, best fit in the subgroup of families of 114 probands without OCD co-diagnosis was for a Mendelian dominant model of transmission whereas a Mendelian additive model of transmission represented best fit in the subgroup of families of 27 probands with an OCD co-diagnosis. Genetic transmission was not shown in those families where the only affected phenotype was ED. The existence of a Mendelian mode of genetic transmission within ED families supports the hypothesis that a common genetic liability could account for both ED and OCD.