We studied the incidence of thyroid function abnormalities observed soon after allogeneic bone marrow transplantations (BMT) and their predictive value on the overall prognosis. Free serum thyroxine, free serum triiodothyronine, total serum reverse triiodothyronine and serum thyrotropin levels were systematically measured in 78 patients before and 3 months after BMT. 41 (52%) had normal hormone levels and 37 (48%) had abnormal ones, among whom four (5%) had peripheral compensated hypothyroidism and 33 (43%) were described as having 'euthyroid sick syndrome' (low thyroxine state, or low T3 syndrome). Two factors strongly influenced the appearance of thyroid abnormalities: steroid dose at the time of thyroid function testing, and age (< or = 16 years/ > 16 years). Among the younger patients, 21 had no thyroid abnormalities, while five did. Among the older patients, 20 had no thyroid abnormalities, while 32 did (P < 0.001). The occurrence of thyroid abnormalities seemed to influence survival strongly, since the 30-month projected survival time was 83% for patients without abnormalities whereas it was 49% for patients with an abnormal profile (P < 0.001). In conclusion, evidence obtained among our population reveals that euthyroid sick syndrome indicates a poor prognosis and that it is very important to monitor thyroid hormone levels (particularly free hormones) soon after allogeneic BMT and regularly thereafter.