To determine how many children with acute lymphoblastic leukemia were initially referred to a pediatric hematologist because of isolated significant thrombocytopenia ([platelet count less than 50,000/mm3] and an otherwise normal complete blood cell count and physical findings), a retrospective review of the Pediatric Oncology Group's charts was undertaken. Review of the records of 2239 children enrolled in the past two acute lymphoblastic leukemia protocols showed that none of these children had significant thrombocytopenia with no other hematologic or physical manifestations of acute lymphoblastic leukemia when they were first seen by the hematologist. The results suggest that routine bone marrow aspiration in the child with isolated thrombocytopenia may be unnecessary to rule out acute lymphocytic leukemia.