We have presented a retrospective study of 129 patients with platelet counts of 1,000 X 10(9)/L or more--72 with myeloproliferative disorders (MPD), and 57 with reactive thrombocytosis (RT)--in an effort to determine the incidence of thrombohemorrhagic complications. Thrombotic disorders occurred with approximately equal frequency in the two groups, being found in three patients (4%) with MPD and three (5%) with RT when the platelet count was over 1,000 X 10(9)/L and in 11% of the patients in each group when the platelet count was less than 1,000 X 10(9)/L. In contrast, bleeding manifestations were more common in MPD, where 36% of patients were affected on one or more occasions versus only 4% of those with RT. However, bleeding was generally not severe, with only six patients requiring transfusions, and three of these patients had been receiving aspirin or corticosteroids when they developed severe gastrointestinal bleeding.