We have investigated the influence of exercise training on natural killer cells and psychological behaviour in cancer patients. 24 women with carcinomas of the breast performed a moderate training program two to three times a week for seven months. At the beginning, after five weeks and at the end of the study we analyzed the amount and the activity of natural killer cells (CD 56) under resting conditions. Personality traits were measured with the FPI-R questionnaire at the same time points. While the amount of natural killer cells did not change, their cytotoxic activity was increased at the end of the study. Furthermore, the discomfort decreased. After 5 weeks satisfaction of life was enhanced and at the end of the investigation we observed a strict connection of this improvement with the frequency of exercise. These results demonstrate a stimulating influence of moderate exercise training on the resting NK-cell activity even in cancer patients.