Erythrocytosis has been observed to occur infrequently in association with large uterine fibroids. Study of this case revealed erythrocytosis prior to surgery and a remission was noted after hysterectomy. It has been suggested that large intraabdominal tumors may interfere with normal pulmonary ventilation with resultant hypoxia which acts as a stimulus to erythropoietin production. An attempt to demonstrate elevated erythropoietin levels in this patient's unconcentrated serum and tumor extracts was unsuccessful. The failure to demonstrate increased erythropoietin activity in this case does not preclude the participation of this humoral mechanism.