One hundred consecutive patients with elevated platelet counts (more than 500,000 per cu. mm.) seen in a 16-month period have been studied. Thirty-six were found to have underlying malignant disease. Fifteen of the patients had recently undergone splenectomy, and four patients had recently had other major surgery. Five of the patients had polycythemia vera, and one patient had chronic granulocytic leukemia. Iron deficiency anemia was associated with thrombocytosis in eight patients. There were 24 patients with miscellaneous benign conditions and seven patients with collagen disorders, usually rheumatoid arthritis. The thrombocytosis of malignant disease was frequently unexpected and was an important clue to the presence of a disseminated malignancy.