In this report, we present data documenting the incidence of reduced REM latency and the lifetime prevalence of psychiatric illness in the parents and siblings of early onset unipolar depressed probands. The prevalence of psychiatric illness (49.3%), especially affective disorders (34.3%), was very high among these relatives. Reduced REM latency in the family members of reduced REM latency probands showed a concordance rate of 70.6% regardless of psychiatric history. The relative risk for unipolar depression among relatives with reduced REM latency was almost three times greater than for relatives with nonreduced REM latency.