OBJECTIVES:  The objectives of this study were to examine the trends of child obesity and changes in BMI over time and to analyze the effect of child obesity on asthma. METHODS:  The Early Childhood Longitudinal Study Kindergarten Class of 1998-99 (ECLS-K) data from the U.S. Department of Education identified 14,188 children in the 3rd grade who were followed through 3 waves (3rd, 5th and 8th grade).  Dichotomized weight group (obese vs. non-obese), based on body mass index (BMI) using the 2000 sex-specific CDC growth charts for the United States, was included in the sample weight-adjusted cross-sectional logistic analyses and a panel fixed effects (FE) model for having asthma. RESULTS:  We detected a gradual increase in the mean BMI and prevalence of obesity over time.  Significant differences were observed for obesity and asthma rate among male and female groups in the 3rd grade (p<0.05).  In cross-sectional analyses, obesity and gender were significantly associated with having asthma for each grade level while adjusting for other demographic covariates (all p<0.05), indicating that children who are obese or male were more likely to have asthma.  Approximately 70% of children stayed within the same weight category from 3rd to 8th grade.  FE model showed a significant time effect where odds of having asthma increased 3.31 times from 3rd to 5th grade while controlling for child obesity (95% CI:  2.89-3.80).  Child obesity and the interaction term between time effect and child obesity had no significant effect on asthma although the sign of estimate for child obesity indicated an increase in risk for asthma. CONCLUSIONS:  We found a risk association between child obesity and asthma in cross-sectional analyses but the effect of obesity on asthma was not significant in a panel data FE model.  Future research with a model using an instrumental variable is warranted to control for endogeneity associated with obesity.