The aim of this study was to determine the effect of radiotherapy on peripheral blood natural killer (NK)-cell number and activity in 15 patients with cancer, prior to the commencement and at the completion of radiotherapy. The following observations were made. (i) Prior to radiotherapy NK activity could not be correlated with the stage of malignancy. (ii) In all patients with advanced disease and with subnormal baseline NK activity, the outcome of radiotherapy was unfavorable. (iii) Following radiotherapy to sites including the mediastinum, patients had decreased NK activity compared with those receiving treatment to other sites. This decrease was not related to the dose of radiotherapy or stage of malignancy. (iv) The tumor response was favorable in most patients whose NK activity decreased as a result of radiotherapy. (v) The decrease in NK activity may be associated with a decrease in the percentage of NK (N901) cells in the peripheral blood. The reduction in NK activity in those patients receiving mediastinal irradiation may be due to the large volume of blood which transits the field, so that the NK cells, or their more radiosensitive precursors, may be damaged and/or differentiation inhibited. Thus, these new observations show that radiotherapy does indeed affect the NK activity in cancer patients predominantly when the irradiation site includes the mediastinum.