The effects of stress, which varies throughout an academic year, on proinflammatory and antiinflammatory cytokines were examined in 44 medical students. This was tested by comparing stimulated cytokines during a baseline period, stress period, and poststress vacation period. During the stress period, compared with the baseline period, levels of IL-6 were reduced, while levels of IL-10 were elevated. During the poststress vacation period, compared with the stress period, levels of IL-6 and TNF-α were increased. However, the changes in stress-related psychological and physiological variables were not significantly associated with changes in levels of proinflammatory and antiinflammatory cytokines. These results suggest that vacation is more likely to have a counterstress effect on proinflammatory cytokines than on an antiinflammatory cytokine and that a stressor may affect changes in immune function independently of self-reported stress.