ESA therapy can increase hemoglobin, decrease blood transfusions, and improve quality of life in patients with chemotherapy induced anemia (CIA). Despite its benefits, ESA therapy increases the risk of venous thromboembolism (VTE) in cancer patients by 50% and can also cause iron restricted erythropoiesis in CIA patients, which may augment the tendency to develop VTE. We postulated that thrombocytosis, a risk factor for VTE in cancer patients, in CIA patients on ESA therapy might be a result of ESA induced iron restricted erythropoiesis. We performed a retrospective analysis of 187 CIA patients who were randomized to receive weekly Epoetin and IV ferric gluconate, oral ferrous sulfate, or no iron for 8 weeks. Nineteen patients experienced 29 VTEs, and patients, whose platelets increased to ≥350,000 cells/uL were three times more likely to experience a VTE (OR 2.9, P = 0.036, logistic regression) with a four times greater incidence of VTE (IRR 4.4, P = 0.001, Poisson regression). Patients treated with IV iron were significantly less likely to develop platelets of ≥350,000 cells/uL (IRR 0.7, P = 0.013, Poisson regression) and had a decreased incidence of VTE. Our study suggests that ESA associated VTE in CIA patients may be, in part, related to the thrombocytosis of ESA induced iron restricted erythropoiesis and may be countered by IV iron.