The purpose of this study was to determine the effect of exercise program on natural killer cell cytotoxic activity(NKCA) in breast cancer patients who had been radiation therapy after surgery. The subjects in the experimental group consisted of 11 breast cancer patients, while the subjects in the control group consisted of 15. Subjects in the experimental group participated in exercise program for 8 weeks. Exercise program consisted of shoulder stretching, arm weight training and treadmill walking exercise. They started to exercise on treadmill for 20 minutes per day, 3 times a week at 40% of maximum heart rate, and increased intensity and duration of exercise so that they were running 30 minutes/day at 60% of maximum heart rate from the 3rd week. to the 8th week. Natural killer cell cytotoxic activity were determined before and after the exercise program. For measuring the natural killer cell cytotoxic activity, 8ml to 10ml blood was collected from the subjects. Mononuclear cell was isolated by centrifuge of the blood and cultured by putting Cr51, and reacted with target cell, K562 cell. Baseline demographic and medical data were compared between groups with the Fisher's exact test and Mann-Whitney U test. For effects of the exercise program, repeated measures ANOVA was used. The result was as follows ; Natural killer cell cytotoxic activity(NKCA) in experimental group comparing with control group significantly increased after the exercise program in case of effector cell : target cell ratio is 100 : 1(p<0.05). The above result suggest that the exercise program for breast cancer patients undergoing radiation therapy after breast surgery may increase the natural killer cell cytotoxic activity.