Background: Although the association between large uterine fibromyomas and secondary polycythemia has been described previously, the mechanism has not been elucidated definitively. Investigators have measured erythropoietin levels in fibromyomas to determine whether these tumors are causing the polycythemia by erythropoietin overproduction; however, these studies were performed before the availability of recombinant erythropoietin assays.Case: A 59-year-old woman presented with a 3-year history of polycythemia. Pelvic examination revealed a large lower abdominal mass. Laboratory evaluation revealed a hemoglobin of 20.8 g/dL, red blood cell mass of 3300 mL, oxygen pressure of 58 mmHg with an oxygen saturation of 89%, and erythropoietin level of 18 mU/mL. Cardiac echocardiogram showed no evidence of shunt. Computed tomography scan of the abdomen showed a large mass arising in the pelvis and compressing both ureters. The patient was treated surgically with a total abdominal hysterectomy. Pathology confirmed a uterine leiomyoma weighing 2320 g. Two months post-surgery, the patient was asymptomatic with a hemoglobin of 13.9 g/dL and erythropoietin level less than 4.0 mU/mL.Conclusion: This case provides evidence for three of the postulated mechanisms by which uterine fibromyomas may cause polycythemia. First, the patient was hypoxic, suggesting shunting within the tumor. Second, the leiomyoma was compressing the ureters, so the kidneys may have been inappropriately producing excess erythropoietin. Third, the tumor itself may have been producing the erythropoietin. In any case, the erythropoietin level in this patient was inappropriately high, providing useful evidence that her polycythemia was secondary to her fibromyoma.