Plasma C‐reactive protein (CRP) concentration, a marker of systemic inflammation, has been shown to be higher in women versus men. We investigated the effect of estrogen (ES) supplementation and moderate intensity endurance exercise in men on serum CRP concentrations. In a randomized, placebo‐controlled, double‐blind, cross‐over design, we measured CRP concentration in 10 men after eight days of either ES supplementation (2 mg 17‐β estradiol/day) or placebo (PL, polycose) prior to, during and following 90 min of cycling at an intensity of 65% VO 2 max. Following a two‐week washout period, the subjects repeated the test after eight days on the alternate treatment. Serum was collected before (0 min), 60 and 90 min during and 2 h after cycling in the postabsorptive state. Subjects maintained similar eating habits and physical activity during both arms of the study. ES supplementation increased serum CRP at all time points by 34–69% as compared with PL (P < 0.001). For PL, CRP at 90 min of exercise was lower than at 0 min (17%, P = 0.066), 60 min (20%, P = 0.013) and 2 h post exercise (18%, P = 0.047). For ES, CRP at 60 min was higher than at 0 min (18%, P = 0.004) and 90 min (11%, P = 0.087) of cycling. We conclude that ES supplementation in men increases CRP concentration, a marker of systemic inflammation, by an average of 50%. In addition, the data suggest that changes in serum CRP during moderate intensity endurance exercise is influenced by sex hormones. (Supported by the Hamilton Health Sciences Foundation, NSERC Canada and National Institute of Nutrition)