We studied the natural killer (NK) cell activity and in vitro production of the cytokines which can enhance NK activity (interleukin-1 beta (IL-1 beta), interferon gamma (IFN gamma), and interleukin-2 (IL-2)) after stimulation in 44 patients with acute leukemia (AL) and 14 normal controls. We also studied the influence of these parameters on relapse and the relapse-free survival (RFS) (after the date of assay) of the AL patients. The NK activity and the production of cytokines in the peripheral blood mononuclear cells (PBMNC) from 16 patients at the untreated or relapsed stage as well as from 12 patients after consolidation were significantly lower than those from controls (both P<0.01), and those from the 16 patients at maintenance or off treatment were also significantly lower than those from the controls (P<0.01 or P<0.05). RFS after the date of assay of the patients in remission with NK activity above the median value was significantly longer than that of the patients below the median (P<0.05). The production of cytokines in the PBMNC from patients who showed continuous complete remission for at least 6 months was higher than that from the patients who relapsed early. These findings suggest that impaired NK cell function and cytokine production are associated with early relapse of AL regardless of remission status.