Studies of the occurrence of mental illness in families have been useful in identifying familial forms of the illnesses and in the development of hypotheses regarding the form and strength of genetic and environmental factors in etiology. Where these major variables are separated by the process of adoption, specific etiologic hypotheses can be tested separately and in combination. A total national sample of 14,500 adult adoptees in Denmark provides the basis of the main components of this research. The research this year has focused on 46 classical schizophrenic adoptees identified in the national sample with a comparable number of control adoptees with no history of mental illness. The remarkable population registers in Denmark permit the identification of the close biological and adoptive relatives of these adoptees. By search of mental hospital registers and ultimately by personal interviews, information on the psychiatric history and status of the relatives has been obtained. During the past year these interviews and other records have been used to obtain consensus diagnoses by two raters without knowledge of the relationship among relatives and adoptees. The first phase of the analysis has now been completed with results which confirm the findings on the first sample (restricted to Greater Copenhagen) that classical chronic schizophrenia occurs almost exclusively among the biological relatives of chronic schizophrenic adoptees and not in their adoptive relatives. A marginal syndrome which has been variously designated as latent or borderline schizophrenia or schizotypal personality was also found to be significantly more prevalent in the biological relatives of chronic schizophrenic adoptees, but was also more prevalent in the biological relatives of adoptees with other major mental disorders or marginal schizophrenia. The study of mental disorder in the relatives of adoptees with affective disorder completes in the previous year has now been published.