47 affectively ill psychiatric patients and their first-, second- and third-degree relatives were investigated by means of an interview and pedigree analysis to determine the incidence of psychiatric illness in their families. The percentage of psychiatric illness appeared greatest in families of bipolar and schizo-affective probands and least in families of unipolar depressives. In addition, we observed that often within a particular family constellation, more than one type of psychiatric illness (i.e., bipolar manic-depression, schizophrenia, alcoholism, etc.) was present. Morbidity risks varied from one affected family to another, indicating that the genetic risk components for some families are greater than for others. These findings are suggestive of multifactorial genetic disease but other genetic models are considered.