The aim of this study was to investigate the contribution of hereditary factors in the development of affective and depressive adjustment disorders. I interviewed 151 index twins with moderately severe and mild affective illness, as well as their co-twins. The analysis of concordance rates indicates that hereditary factors may be important in the development of bipolar disorder and in major depression, except in non-psychotic, hysterical individuals. Furthermore, hereditary factors may not play any role in dysthymic disorder and depressive adjustment disorder. These findings are tentative and should be viewed against the methodologic limitations of this study, which include small sample size of the subgroups and the use of the Present State Examination as the basis for the DSM-III diagnoses.