We investigated the association between cardiorespiratory fitness (CRF) and carotid atherosclerosis in 9 871 Korean men aged 40-81 years. We measured carotid intima-media thickness by using B-mode ultrasonography, and cardiorespiratory fitness was measured by a maximal treadmill exercise test using the Bruce protocol. Carotid atherosclerosis was defined as an intima-media wall thickness greater than 1.2 mm or stenosis >25% of carotid arteries, while CRF was classified as low fit (<20%), moderately fit (20-<60%), or high fit (> or =60%) categories based on age-specific VO (2peak) (ml/kg/min) percentiles. The presence of carotid atherosclerosis across CRF categories was 11.7% (low fit), 9.6%, and 7.7%, respectively. After adjustment for age, cigarette smoking, body mass index, physical activity, there was an inverse association between CRF and carotid atherosclerosis (p for trend <0.001). The odds ratio of presence of carotid atherosclerosis in the high-fit men versus the low-fit men was 0.67 (95% CI, 0.55, 0.80). After additional adjustment for high cholesterol, hypertension, and diabetes mellitus, the results remain unchanged and the odds ratios across CRF levels were (95% CI): 1.00 (low fit, referent), 0.85 (0.71, 1.01), 0.71 (0.59, 0.85) (p for trend <0.001), respectively. High levels of cardiorespiratory fitness were associated with a lower risk of having carotid atherosclerosis in middle-aged and elderly men.