Iron deficiency anemia is a cause of reactive thrombocytosis. A moderate increase in platelet numbers is common but sometimes counts may exceed 1,000 x 10(9)/1. The mechanisms causing reactive thrombocytosis are unclear. In this study, we evaluated 15 women with iron deficiency anemia and thrombocytosis (platelets >450 x 10(9)/1) and 16 women with iron deficiency anemia with normal platelet counts. Serum samples were taken before oral iron replacement therapy, after 1 and 3 months and at the end of replacement therapy. Thrombopoietin, erythropoietin (EPO), leukemia inhibitory factor, interleukin-6 and interleukin-11 levels were assayed. There was no change in the levels of thrombopoietic cytokines except for EPO. The correlation between high EPO levels and high platelet counts may suggest that EPO increases platelet counts, but the same EPO revel changes can also be demonstrated in women with iron deficiency anemia but normal initial platelet counts. The fact that the levels of other cytokines remained unchanged during treatment suggests that either these cytokines have no effect on reactive thrombocytosis or the change in platelet counts in our patients is in a narrow range and is thus not affected by the cytokine levels. Copyright (C) 2000 S. Karger AG, Basel.