The objective of the study was to determine whether the frequency and severity of sleep problems were greater in children of affectively ill mothers than in children of control mothers. Sleep problems were studied in children of mothers with a diagnosis of unipolar (N = 38) and bipolar (N = 23) affective illness and children of mothers with no current or past psychiatric diagnosis (N = 24). Mothers' reports on the Child Behavior Checklist (CBCL) were obtained three times, 4 years apart, on sibling pairs (ages 1.5-3.5 and 5-8 years, respectively, at first assessment). In addition, on the third assessment, the Diagnostic Interview for Children and Adolescents was filled out by mothers and children. In both siblings, sleep problems, as assessed through the CBCL, were more frequent and severe in children of affectively ill mothers. In younger siblings, the persistence of sleep problems was more frequent in children of affectively ill mothers. Co-occurrence of sleep problems among siblings was more frequent in children of affectively ill mothers than in those of control mothers.