The aim was to investigate the association between breakfast consumption and long-term weight gain in an adult male population. We evaluated prospective data on 20,064 U.S men, 46 to 81 years of age, who participated in the Health Professionals Follow-up Study. Data on body weight, dietary factors, and lifestyle variables were obtained by validated questionnaires. We examined weight gain during 10 years of follow-up. Overall, 5,857 men had a weight gain of 5 kg or greater during 10 years of follow-up. Breakfast consumption was inversely associated with the risk of 5-kg weight gain after adjustment for age [hazard ratio (HR) = 0.77 (95% confidence interval [CI], 0.72 to 0.82)], and this association was independent of lifestyle and BMI at baseline [HR = 0.87 (95% CI, 0.82 to 0.93)]. Fiber and nutrient intakes partially explained the association between breakfast consumption and weight gain. The inverse association between breakfast consumption and weight gain was more pronounced in men with a baseline BMI of 25 kg/m(2) or lower [multivariate HR = 0.78 (95% CI, 0.70 to 0.87)] than in men who were overweight at baseline [HR = 0.92 (95% CI, 0.85 to 1.00)]. Furthermore, we observed that an increasing number of eating occasions in addition to three standard meals was associated with a higher risk of 5-kg weight gain [HR = 1.15 (95% CI, 1.06 to 1.25, for >or=2 vs. 0 additional eating occasions)]. These findings suggest that the consumption of breakfast may modestly contribute to the prevention of weight gain as compared with skipping breakfast in middle-aged and older men.