Thyroid dysfunction has been reported following single dose and fractionated radiation in the context of bone marrow transplantation (BMT). Limited data are available regarding this complication following hyperfractionated radiation. We undertook a retrospective analysis of thyroid function in 150 patients who received BMT at our institution, and who were alive and disease-free for at least 1 year after transplant. There were 100 pediatric patients and 50 adult patients, with a median follow-up of 6.2 years for the whole group. These patients had acute (n = 91) or chronic leukemias (n = 36), severe aplastic anemia (n = 18) or immunodeficiency disorders (n = 5). The majority of the patients received radiation-based cytoreductive regimens including 129 patients who received hyperfractionated total body irradiation (TBI) to a total dose of 1375 cGy or 1500 cGy and 10 patients who received total lymphoid irradiation (TLI) to a total dose of 600 cGy. Twenty two patients of the cohort of 150 patients (14.7%) and 21 of the 139 patients (15.1%) who received hyperfractionated radiation were found to have developed hypothyroidism, 11-88 months after transplant (median 49 months). Eight patients had received 1375 cGy and 12 patients 1500 cGy TBI, while one patient was treated with 600 cGy TLI and one patient was treated with chemotherapy only (busulfan and cyclophosphamide). Three patients had primary thyroid failure with an elevated TSH and a low T4 index, while 19 patients had compensated hypothyroidism with an elevated TSH but a normal T4 index. Six of eight patients with untreated compensated hypothyroidism recovered spontaneously.(ABSTRACT TRUNCATED AT 250 WORDS)