Thyroid function studies were followed serially in 27 long-term survivors (median 33 months) of bone marrow transplantation. There were 15 men and 12 women (median age 13 1/12 years, range 11/12 to 22 6/12 years). Aplastic anemia (14 patients) and acute nonlymphocytic leukemia (eight patients) were the major reasons for bone marrow transplantation. Pretransplant conditioning consisted of single-dose irradiation combined with high-dose, short-term chemotherapy in 23 patients, while four patients received a bone marrow transplantation without any radiation therapy. Thyroid dysfunction occurred in 10 of 23 (43 percent) irradiated patients; compensated hypothyroidism (elevated thyroid-stimulating hormone levels only) developed in eight subjects, and two patients had primary thyroid failure (elevated thyroid-stimulating hormone levels and low T4 index). The abnormal thyroid studies were detected a median of 13 months after bone marrow transplantation. The four subjects who underwent transplantation without radiation therapy have remained euthyroid (median follow-up two years). The only variable that appeared to correlate with the subsequent development of impaired thyroid function was the type of graft-versus-host disease prophylaxis employed; the irradiated subjects treated with methotrexate alone had a higher incidence of thyroid dysfunction compared to those treated with methotrexate combined with antithymocyte globulin and prednisone (eight of 12 versus two of 11, p less than 0.05). The high incidence and subtle nature of impaired thyroid function following single-dose irradiation for bone marrow transplantation are discussed.