After briefly reviewing the main findings of twin research in. functional psychoses, the author focuses the attention on neglected aspects of twin research, such as the study of co-twins of monozygotic (MZ) subjects with functional psychosis, and the study of life histories of discordantly affected twins. Through a thorough investigation of the nonpsychotic co-twins of MZ subjects, one might be able to identify vulnerability factors. By studying the life course of the twins in a pair, discovery of environmental etiological factors are possible. By including twins with both schizophrenic, manic-depressive and reactive psychotic symptomatology, one should be able to discover whether the environmental factors are of a specific kind.