It is known that in severe nonthyroidal illness the regulation of thyroid function, the distribution and metabolism of thyroid hormones may change. The present study aimed at clarifying whether a change in the function of the pituitary-thyroid axis can be detected in an approximately homogeneous group of haematological patients, and how it is correlated with the various phases of the disease and with the therapeutic result. Studies were performed on patients with chronic and acute myelogenous leukaemia: serum levels of total thyroxine and triiodothyronine, free thyroxine and triiodothyronine, reverse triiodothyronine and thyrotropic hormone were determined. Apart from a few cases, there was no dysfunction of the pituitary-thyroid axis in chronic leukaemic patients being in the remission phase. However, the peripheral thyroxine metabolism may be altered. The longitudinal studies on acute myelogenous leukaemic patients indicate that, with the progression of the disease, serum TSH and thyroid hormone levels were reduced in a part of the cases and it is not justified to assess the free serum thyroxine level by an analogue-tracer method in this disease. The examinations have revealed that the various phases of the clinical picture as well as the therapeutic results considerably influence the function of the pituitary-thyroid axis. It seems reasonable to consider these findings in the other severe nonthyroidal illnesses as well.