A retrospective chart review of six African-American children with a diagnosis of macrothrombocytopenias (MTCP) was performed to evaluate the accuracy of their diagnosis. The following was diagnosed in the six children with MTCP: Fechtner syndrome (two children), Sebastian syndrome (one child), and unnamed MTCP (three children). In five families, chronic idiopathic thrombocytopenic purpura (ITP) was diagnosed in the propositus, which resulted in therapy using steroids, intravenous immunoglobulin (IVIG), and in one case splenectomy. Bleeding symptoms were generally mild. All six patients had thrombocytopenia ranging from 10 to 125 x 10(9)/L with mean platelet volume of 8 to 20 fL. Bleeding times were abnormal in two of three patients, and platelet aggregation was abnormal in three of four patients tested. Bone marrow aspirates were reported as increased megakaryocytes in the three patients on whom the procedure was performed. Ultrastructural morphology of platelets and leukocytes was performed in all six patients demonstrating giant platelets in all six patients and leukocyte inclusions in three patients. Differentiating MTCP from the more common ITP can be difficult but important in avoiding unnecessary diagnostic studies and potentially harmful therapy associated with ITP.