Marathon runners (N = 22) who had completed at least seven marathons (X ± SEM = 23.6 ± 5.7) and had been training for marathon race events for at least 4 yr (12.3 ± 1.3) were compared with sedentary controls (N = 18). Although the two groups were of similar age (38.7 ± 1.5 and 43.9 ± 2.2 yr, respectively) and height, the marathon runners were significantly leaner and possessed a VO2max 60% higher than that of the controls. Neutrophil counts tended to be lower in the group of marathoners, while other leukocyte and lymphocyte subsets were similar to controls. Mitogen-induced lymphocyte proliferation did not differ between groups. Natural killer cell cyto-toxic activity (NKCA) was significantly higher in the marathoners versus controls (373 ± 38 vs 237 ± 41 total lytic units, respectively, a 57% difference, P = 0.02). For all subjects combined (N = 40) and within the group of marathon runners (N — 22), percent body fat was negatively correlated with NKCA (r = -0.48, P = 0.002; r = -0.49, P = 0.019, respectively), and age was negatively correlated with Con A-induccd lymphocyte proliferation (r = -0.41, P = 0.009; r = -0.53, P = 0.011, respectively). These data indicate that NKCA but not mitogen-induced lymphocyte proliferation is higher in marathon runners relative to sedentary controls.