Intense exercise affects the immune system. This study examines effects of three consecutive days of 1 h high-intensity exercise on lymphocyte counts, oxidative DNA damage, and apoptosis in young untrained (n = 8, 23.8 +/- 3.2 years; UT) and endurance-trained (n = 8, 21.1 +/- 3.7 years; TR) subjects. The subjects performed cycle ergometer exercise at 75% VO2max 1 h daily for three consecutive days (exercise session). Blood samples were collected before exercise on the first day of the exercise session (day 1, D1) and at 24 h after the session (day 4, D4). Total lymphocyte counts, a lymphocyte oxidative DNA damage index using Comet assay with human 8-oxoguanine DNA glycosylase, oxidative stress markers, and apoptosis markers were measured. Lymphocyte counts at D1 in TR were significantly lower than in UT. Lymphocyte counts in TR changed little at D4 (from 1,988 +/- 475 to 1,854 +/- 363 cell/microl), but the lymphocyte counts in UT decreased significantly at D4 (from 2,583 +/- 564 to 1,911 +/- 528 cell/microl, P < 0.05). Lymphocyte oxidative DNA damage increased concomitantly with exercise sessions in both the groups (UT, from 31.3 +/- 17.5 to 48.9 +/- 15.7%; TR, from 21.9 +/- 5.2 to 62.1 +/- 12.5%, P < 0.05). Although no change was found in apoptosis markers over time, Annexin-V(+) cells decreased in TR (effect size D = 0.8 is large). Three consecutive days of 1 h exercise decreased lymphocyte counts with increased lymphocyte oxidative DNA damage in UT. Lymphocyte counts remained unchanged irrespective of increased oxidative DNA damage in TR. Decreased lymphocyte apoptosis might prevent the decrease of lymphocytes in TR.