AIMS: To analyze the relationships between physical fitness, lifestyle-related factors, and obesity in a large population of children. METHODS: A cross-sectional study design including children aged 7-12 years (n = 715) was used. Adiposity measures included subcutaneous fat mass (SFM) and body mass index (BMI). Physical fitness and lifestyle-related factors were also assessed. RESULTS: When SFM was used as the adiposity variable, the odds ratios (OR) for being obese in boys in the highest quartiles of fitness were 0.02 (95% CI 0.02-0.13) for aerobic fitness, 0.04 (95% CI 0.01-0.16) for dynamic force, and 5.32 (95% CI 1.82-15.58) for running speed (in which quartile 1 corresponds to the best performance) compared with boys in the lowest quartile. In girls, the OR for those in the highest quartiles of fitness were 0.04 (95% CI 0.01-0.14), 0.16 (95% CI 0.05-0.51), and 5.24 (95% CI 1.74-15.75), respectively, showing a significant dose-response relationship between fitness and fatness in both sexes (p for trend <0.001). CONCLUSIONS: An inverse relationship between physical fitness levels and the risk of being overweight/obese was found inasmuch as children with higher physical fitness seem to be more protected against fat mass accumulation than their counterparts with lower fitness levels.