We studied three children with chronic idiopathic thrombocytopenic purpura (ITP) and their immediate families. All three patients and 10/13 family members manifested at least one immunologic defect, eg, decreased numbers of T lymphocytes (1/3, 5/13), diminished in vitro response to phytohemagglutinin (2/3, 6/13), dysgammaglobulinemia (2/3, 4/13), altered autoantibodies (1/3, 5/13), and decreased serum properdin levels (3/3, 2/13). In addition, one parent and two asymptomatic siblings of two of the propositi had shortened platelet life spans with normal platelet counts. The HLA antigens A3 and B7 were identified in all three families. There also appeared to be an association between a familial haplotype and the immunologic defects. Chronic ITP appears to occur in families with underlying immunologic defects, which are genetically related.