Thyroid function abnormalities in 270 adult patients post-BMT are described. Various conditioning regimens were used and the effects of three TBI and one chemotherapy only based regimens are compared. The overall incidence of elevated TSH is 8.9; 3.8, 7.2 and 16.7% in those patients who received 300, 500 and 1200 cGy respectively and 11.7% in those who received BuCy conditioning. Three cases (1.1%) of clinial hypothyroidism were observed. Compensated hypothyroidism defined as an elevated TSH in the presence of normal T3, T4 levels and transient in some cases, was the most common finding. All but four cases occurred in the first 2 years after BMT. In the remaining four, three occurred in patients with chronic GVHD. The results reported here show a lower prevalence than observed in most other reviews, particularly for children. A trend was observed with increasing radiation doses. The results are not significantly different from those we observed in the BuCy regimen.