The predisposition of immunodeficient patients to the development of peripheral blood abnormalities is well documented. Less often recognized is the presence of immune defects in patients who initially present with "idiopathic" forms of thrombocytopenic purpura or autoimmune hemolytic anemia in the absence of symptomatology indicative of an immuno-deficiency. In this report we describe two children, each of whom had intermittent and at times severe hemolysis and thrombocytopenia secondary to formation of autoantibodies. Although the initial clinical impression was the presence of an "idiopathic" form of platelet or red cell destructive phenomenon, subsequent observations demonstrated abnormalities of the immune system which affected both B and T lymphocytes. In one of the patients there was a marked deficiency of IgA with a moderate but progressive decrease in IgM and IgG. The other patient had a moderate decrease in IgA and a failure to sensitize to dinitrochlorobenzene. Both exhibited dysmorphic features, including small stature, abnormal dentition, hyperelasticity of the skin, and hyperextensibility of the joints. Neither had significant problems with bacterial, fungal, or viral infections. These observations highlight the importance of evaluating the immune system in children with "idiopathic" forms of hemolytic anemia or thormbocytopenia. Demonstration of these abnormalities can have important therapeutic implications and may also provide information regarding the etiology of the blood abnormalities.