OBJECTIVE: To test the hypothesis that stress reducing techniques such as meditation alter immune responses after strenous physical stress. METHODS: The hypothesis was tested by studying six meditating and six non-meditating male runners in a concurrent, controlled design. After a period of six months with meditation for the experimental group, blood samples were taken immediately before and after a maximum oxygen uptake test (VO2max). RESULTS: The increase in CD8+ T cells after VO2max was significantly less in the meditation group than in the control group (P = 0.04). The amount of CD2+ cells doubled after VO2max, mainly because of a rise in the CD8+ fraction. CONCLUSIONS: Meditation may modify the suppressive influence of strenous physical stress on the immune system.