I investigated the contribution of hereditary factors in somatoform disorders. Fourteen monozygotic and 21 dizygotic index twins and their co-twins were personally interviewed. The results showed a concordance of 29% in monozygotic and 10% in dizygotic pairs. However, similarity in childhood experience seemed to influence the concordance rates. Thus, even if somatoform disorders appear familial, the transmission may be environmental. Furthermore, the study showed a high frequency of anxiety disorders, especially generalized anxiety disorders, in the co-twins of somatoform-disordered twins.