Objective To evaluate relationships between type of milk consumed and weight status among preschool children. Design Longitudinal cohort study. Setting The Early Childhood Longitudinal Study, Birth Cohort, a representative sample of US children. Participants 10 700 US children examined at age 2 and 4 years. Main outcome measures Body mass index (BMI) z score and overweight/obese status as a function of milk type intake. Results The majority of children drank whole or 2% milk (87% at 2 years, 79.3% at 4 years). Across racial/ethnic and socio-economic status subgroups, 1%/skim milk drinkers had higher BMI z scores than 2%/whole milk drinkers. In multivariable analyses, increasing fat content in the type of milk consumed was inversely associated with BMI z score (p<0.0001). Compared to those drinking 2%/whole milk, 2- and 4-year-old children drinking 1%/skim milk had an increased adjusted odds of being overweight (age 2 OR 1.64, p<0.0001; age 4 OR 1.63, p<0.0001) or obese (age 2 OR 1.57, p<0.01; age 4 OR 1.64, p<0.0001). In longitudinal analysis, children drinking 1%/skim milk at both 2 and 4 years were more likely to become overweight/obese between these time points (adjusted OR 1.57, p<0.05). Conclusions Consumption of 1%/skim milk is more common among overweight/obese preschoolers, potentially reflecting the choice of parents to give overweight/obese children low-fat milk to drink. Nevertheless, 1%/skim milk does not appear to restrain body weight gain between 2 and 4 years of age in this age range, emphasising a need for weight-targeted recommendations with a stronger evidence base.