A patient with ataxia telangiectasia presenting with a severe recurrent bleeding diathesis characterized by a prolonged bleeding time, normal platelet counts and clot retraction, absent platelet aggregation, and normal platelet factor 3 availability is described. These findings are indicative of a thrombasthenic-like pattern associated with multiple membrane receptor site defects. Chromosomal studies revealed a 14/14 tandem translocation involving chromosomal band 14q32 in peripheral T-lymphocytes; this chromosomal marker was not found in peripheral B-lymphocytes, direct bone marrow preparations, or skin fibroblasts.We postulate that the platelet functional defect demonstrated in this patient occurred in a clone of abnormal platelet stem cells possibly containing the chromosomal marker. This defect could be analogous to the situation in chronic myelogenous leukemia in which similar platelet functional disorders have been noted and in which the marker Philadelphia chromosome has been present on megakaryocytes. Our patient would also appear to be at high risk for the development of a T-cell malignancy.