The thyroid function was evaluated before and after 6 months of recombinant human erythropoietin (rhEPO) treatment (1,500-9,000 U/week) in 22 hemodialysis patients with hematocrit levels 5% (group I) and 11 patients with an increase < 5% (group II). Before rhEPO administration, the levels of thyroid hormones, especially free thyroxine (T4) and free triiodothyronine (T3), were below the normal range despite normal thyrotropin values in most of the patients (low T4:7 cases in group I and 9 in group II; low T3:10 cases in group I and 10 in group II). RhEPO treatment significantly increased both total amount and free fractions of thyroid hormones in group I, whereas it did not affect these values in group II. Consequently, the pretreatment low T4 or low T3 status was resolved in a substantial number of the patients in group I (low T4:5 cases, low T3:4 cases). In addition, there was a significant correlation between the increases in hematocrit and free T3 in all studied subjects (r = 0.603; p < 0.05). These results suggest that anemia may participate to some extent in the pathogenesis of thyroid dysfunction in hemodialysis patients with renal anemia.