Background: Milk is promoted as a healthy beverage for children, but some researchers believe that estrone and whey protein in dairy products may cause weight gain. Others claim that dairy calcium promotes weight loss. Objective: To assess the associations between milk, calcium from foods and beverages, dairy fat, and weight change over time. Design, Subjects, and Outcome Measure: We followed a cohort of 12 829 US children, aged 9 to 14 years in 1996, who returned questionnaires by mail through 1999. Children annually reported their height and weight and completed food frequency questionnaires regarding typical past-year intakes. We estimated associations between annual change in body mass index (BMI) (calculated as weight in kilograms divided by height in meters squared) and our dietary factors, adjusted for adolescent growth and development, race, physical activity, inactivity, and (in some models) total energy intake. Results: Children who drank more than 3 servings a day of milk gained more in BMI than those who drank smaller amounts (boys: β±SE, 0.076±0.038 [P=.04] more than those who drank 1 to 2 glasses a day; girls: β±SE, 0.093±0.034 [P=.007] more than those who drank 0 to 0.5 glass a day). For boys, milk intake was associated with small BMI increases during the year (β±SE, 0.019±0.009 per serving a day; P=.03); results were similar for girls (β±SE, 0.015±0.007 per serving a day; P=.04). Quantities of 1% milk (boys) and skim milk (girls) were significantly associated with BMI gain, as was total dietary calcium intake. Multivariate analyses of milk, dairy fat, calcium, and total energy intake suggested that energy was the most important predictor of weight gain. Analyses of year-to-year changes in milk, calcium, dairy fat, and total energy intakes provided generally similar conclusions; an increase in energy intake from the prior year predicted BMI gain in boys (P=.003) and girls (P=.03). Conclusions: Children who drank the most milk gained more weight, but the added calories appeared responsible. Contrary to our hypotheses, dietary calcium and skim and 1% milk were associated with weight gain, but dairy fat was not. Drinking large amounts of milk may provide excess energy to some children.