Abstract
A nine‐year‐old black girl with essential thrombocythemia developed hemoptysis. Only two other cases in the English literature have been described. Ultrastructure and functional characteristics of this patient's platelets were studied. Twenty‐six percent of the patient's platelets were very large (megathrombocytes). Spontaneous aggregates from the patient's platelets were not compact, and the pseudopods did not interdigitate. Both qualitative and quantitative defects in platelet organelles were detected. The microtubular system was faulty in organization. Furthermore, the number of granules (especially alpha granules) was reduced. Platelet aggregation studies demonstrated subnormal aggregation in response to ADP, epinephrine, and collagen, but aggregation with ristocetin was normal. It is postulated that a platelet membrane abnormality may be the cause of their defective platelet aggregation.