In patients with ovarian cancer and thrombocytosis, tumors may express interleukin-6, which stimulates production of thrombopoietin. The increase in platelets is associated with more rapid disease progression. Therapies that lower platelet counts may enhance antitumor effects of other agents. Platelets are highly reactive cellular effectors of hemostasis, immunity, and inflammation. 1 The concept that platelets play key roles in cancer growth and metastasis is long-standing. In fact, the clinical observation that thrombocytosis (defined as a platelet count of >450,000 per cubic millimeter) occurs in patients with solid tumors was made more than 100 years ago. 2 , 3 Nearly 40% of persons incidentally found to have platelet counts exceeding 400,000 per cubic millimeter in the absence of iron deficiency and benign inflammatory conditions have an occult cancer, most commonly a primary gastrointestinal, lung, breast, or ovarian cancer. 4 Beyond clinical observations, experimental evidence . . .