The understanding and treatment of schizophrenia have been greatly hampered by uncertainty about etiological factors and controversy about diagnostic boundaries. Adoption studies have helped clarify both of these problems. First, the adoptees study method, the index offspring of adopting-away schizophrenic biological parents are compared with control adoptees whose adopting-away parents were non-schizophrenic. Earlier studies have found increased illness in the index adoptees, thus supporting a genetic hypothesis. Additionally, in this approach the comparison of index versus control adoptive rearing families permits the assessment of environmental joint or interaction effects. An on-going Finnish study appears to provide evidence supporting both genetic and environmental main effects, as well as joint effects. Secondly, in the adoptees' relatives method, the biological relatives of schizophrenic index adoptees are compared with the biological relatives of non-schizophrenic control adoptees. A Danish study found that the relatives not only have more frequent typical, narrowly defined schizophrenia but also have more 'latent', non-psychotic forms of the illness. Thus, the adoption studies of schizophrenia are proving valuable in establishing the significance of both genetic and environmental contributions to the illness and in clarifying the diagnostic criteria for a genetically relevant syndrome.