OBJECTIVE To compare children's actual weight status with their parents' perceptions of their weight status. DESIGN Cross-sectional study, including a self-administered questionnaire. SETTING Seven elementary schools in Middlesex-London, Ont. PARTICIPANTS A convenience sample of pupils in grades 4 to 6 and their parents. Of the 770 child-parent pairs targeted, 355 pairs participated in the study. MAIN OUTCOME MEASURES Children's weight, height, and body mass index (BMI). Parents' perceptions of their children's weight status, family demographics, and parents' self-reported body weight and height. The United States Centers for Disease Control's BMI-for-age references were used to define children's weight status (underweight, overweight, or obese). RESULTS Response rate was 46%. Children's actual weight status (ie, 29.9% overweight or obese and 1.4% underweight) was different from their parents' perceptions of their weight status (ie, 18.3% overweight or obese and 17.2% slightly underweight or underweight). Factors such as children's sex and ethnicity and mothers' weight influenced parents' ability to recognize their children's weight status. Parents' misperceptions of their children's weight status seemed to be unrelated to their levels of education, their family income, or their children's, ages. CONCLUSION A large proportion of parents did not recognize that their children were overweight or obese. Effective public health strategies to increase parents' awareness of their children's weight status could be the first key steps in an effort to prevent childhood obesity.