Three-day food records from a large group of marathon runners (291 men, 56 women) were analyzed and compared to various standards of dietary quality. Intake by the runners exceeded two-thirds of the Recommended Dietary Allowance for all nutrients except vitamin D and zinc in female runners. Energy and nutrient intake levels in the marathon runners were higher than those of the general U.S. population except for total fat, vitamin B-12, and zinc in both men and women and sodium in men. In general, runners consumed better diets than the general population. However, both caloric intake and percent energy as carbohydrate were lower than recommended for individuals engaging in endurance exercise. The need for professional help in planning high-carbohydrate diets for runners is indicated. Increasing amounts of training were not associated with significant increases in nutrient densities across three activity categories within this group of runners. However, more than 75% of the runners perceived their diets to be much improved after they began regular training (men 8.2 +/- 0.3 years of running experience, women 6.7 +/- 0.6). Together, these data suggest that regular moderate-to-heavy endurance exercise is associated with a better quantity and quality of nutrient intake than that found in the general population, which may in part be related to the adoption of such exercise by previously sedentary individuals.