OBJECTIVE Between 1975 and 1990, 13 patients seen at The Hospital for Sick Children, Toronto, Ontario, were noted to have concurrent idiopathic thrombocytopenia and neutropenia (ITN; platelet count < 150 x 10(9)/L; absolute neutrophil count < 1.5 x 10(9)/L). These patients all had normal marrow function and no evidence of systemic disease. METHODS A detailed chart review was performed to define the clinical and laboratory features of these patients. RESULTS Although this was a heterogeneous group of patients, they shared several common characteristics. The disease followed a chronic course with thrombocytopenia or neutropenia or both that persisted or recurred over the entire period of follow-up in all patients (18 months to 15 years). The disease was associated with splenomegaly in eight patients and lymphadenopathy in nine patients. Boys were affected more frequently than were girls (ratio 11:2). Thrombocytopenia improved temporarily during treatment with corticosteroids, i.v. immunoglobulin G, RhoGAM, and a variety of immunosuppressive agents; however, neutropenia tended to be much more resistant to these therapies. Splenectomy was ineffective in two children in whom the procedure was performed. Platelet of and neutrophil antibodies or both were detected in five of six patients who were tested, suggesting an autoimmune cause for the cytopenias. CONCLUSIONS These findings suggest that ITN in childhood is distinct from immune thrombocytopenic purpura, particularly in terms of its chronicity and poor response to therapy.