OBJECTIVE A child who was extensively evaluated for polycythemia is reported. Polycythemia, or erythrocytosis, is seen rarely in children. The mechanisms for congenital and/or familial erythrocytosis are discussed. METHODS A 10 1/2-year-old white girl was referred for evaluation of polycythemia, which was detected incidentally during an emergency room visit for a febrile illness. She underwent extensive evaluation to determine the cause of the polycythemia. The literature was reviewed to determine the occurrence of congenital and/or familial erythrocytosis in children and its various causes. RESULTS Despite extensive evaluation, no specific cause of the erythrocytosis could be determined in our patient. The erythrocytosis appeared to be secondary to an inappropriately elevated serum erythropoietin concentration. Serum erythropoietin rose further after phlebotomy, suggesting nonautonomous hypersecretion. After a review of the literature, we hypothesize that she had an inappropriate erythropoietin expression related to an abnormality in the renal oxygen-sensing mechanism governing erythropoietin synthesis. CONCLUSIONS A discussion of congenital and familial erythrocytosis is presented, and a review of the literature regarding the possible mechanisms causing erythrocytosis is included.