We have followed for 33 months the changes that occurred in natural killer (NK) cell numbers and activity in a patient (A) with hairy cell leukaemia (HCL), using a single cell assay and a microcytotoxicity assay. The composition of the peripheral blood mononuclear cell population and malignant cell phenotype were also analysed. During this period he received treatment with interferon and his grossly enlarged spleen was removed. Four further patients were also studied, two were splenectomized and all had received treatment with interferon. In four of the five patients studied there was an apparent link between low NK activity and presence of a tumour-infiltrated spleen, and in the fifth patient, who was aleukemic and had no splenomegaly, NK function was related to disease activity. There was no correlation between NK activity and the number of target binding (TB) cells in these five patients. IFN had little direct effect on overall NK activity, but the proportion of killing cells among TB cells was increased. Three patients showed binding of several cells to a single target. Further analysis revealed that in the patients most of the TB cells were not CD56-positive NK cells, in contrast to TB cells from normal subjects. In patient A a large proportion (84%) of TB cells were identified as malignant cells and in patient E 15% of TB cells were malignant cells. The phenotype of the malignant cells was: CD19+, HLA-DR+ and CD25(Tac)+, except for patient A. In this patient the hairy cells were positive for the NK marker CD56 as well as the monocyte marker CD14. Furthermore, a change occurred in phenotype as only later samples carried CD25. It is concluded that the level of NK function correlates closely with disease activity in HCL and that competitive target cell binding by malignant cells may be one cause of depressed NK-cell function in hairy cell leukaemia.