We used the Amersham radioimmunological method to measure plasma 1 alpha 25-dihydroxyvitamin D3 (calcitriol) levels in patients presenting with one of the following diseases: (i) non-acute myelodysplastic syndrome (39 cases); (ii) acute myeloid leukaemia in blastic phase (43 cases) or in complete remission (15 cases) and (iii) acute lymphoid leukaemia in blastic phase (11 cases). All patients had normal metabolic functions. Compared with our standard laboratory values (15-35 pg/ml), the results of these assays were related to the type of pathology or, in patients with acute myeloid leukaemia, to the stage of the disease (p less than 0.001). Moreover, the mean plasma calcitriol values differed according to the type of pathology (p less than 0.003). Patients with acute myeloid leukaemia in blastic phase had a low level of calcitriol as compared with controls (p less than 0.05) and with patients with acute myeloid leukaemia in complete remission (p less than 0.001) whose calcitriol levels were never low. In contrast, there was no significant difference between controls and patients with myelodysplastic syndrome or acute lymphoid leukaemia in blastic phase. This study demonstrates the usefulness of plasma calcitriol assays in malignant blood diseases where low values in certain types of leukaemia would incite to include calcitriol in therapeutic regimens.