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 < 25%. Based upon the changes in hematocrit following rhEPO treatment, the patients were divided into two groups: 11 patients with an increase of the hematocrit level > 5% (group I) and 11 patients with an increase < 5% (group II). Before rhEPO administration, the levels of thyroid hormones, especially free thyroxine (T<sub>4</sub>) and free triiodothyronine (T<sub>3</sub>), were below the normal range despite normal thyrotropin values in most of the patients (low T<sub>4</sub>:7 cases in group I and 9 in group II; low T<sub>3</sub>: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 T<sub>4</sub> or low T<sub>3</sub> status was resolved in a substantial number of the patients in group I (low T<sub>4</sub>:5 cases, low T<sub>3</sub>:4 cases). In addition, there was a significant correlation between the increases in hematocrit and free T<sub>3</sub> 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.