Natural killer (NK) cell activity was studied serially in the peripheral blood obtained from 35 patients with recurrent aphthous ulcers (RAU) and from 46 age/sex-matched normal healthy controls. The NK cell activity was assayed by a 4-hr 51Cr release assay using K562 cells as targets. The results showed that the patients in remission (2 weeks of convalesence) had normal NK cell activity compared to that of normal controls. Four stages of evolution (early, exacerbation, postexacerbation, and convalescence) in these patients were further evaluated. Increased NK cell activity in the exacerbation of major aphthous ulcer was noted. In contrast, depressed NK cell activity in postexacerbation and in 1 week of convalescence was found at different effector/target (E/T) ratios. The depressed NK cell activity was interpreted as temporary and a secondary phenomenon in the immunopathogenesis of this disease. Partial restoration of depressed NK cell activity by adding recombinant interleukin-2 (rIL-2) suggests that other factors are also involved in the process or that IL-2 deficiency exists in RAU patients. However, no deficiency of plasma IL-2 and interferon-gamma (IFN-gamma) was noted. The percentage of large granular lymphocytes (LGL, NK/K) was correlated with NK cell activity, with both parameters being depressed in the postexacerbation of RAU patients. NK cell activity of RAU patients was still depressed after the depletion of plastic adherent cells. Suprisingly, in contrast to the remission stage, unresponsiveness to rIL-2 of normal NK cell activity in the exacerbation stage was found. The discrepancy was not associated with different subpopulations of NK heterogeneous cells. Immunopathogenesis of RAU on the NK-IFN-IL-2-IL-2R system needs further clarification.