Context: Childhood overweight is a global health problem. Monitoring of childhood body mass index (BMI) may help identify critical time periods during which excess body weight is accumulated. Purpose: To examine changes in mean BMI and the prevalence of at-risk-for overweight in repeated cross-sectional samples of rural first grade schoolchildren between 1999 and 2004. Methods: BMI was determined in 479 first graders from a rural Wyoming school district. BMI and gender-specific BMI-for-age percentiles were determined and evaluated over the 6 years. Children were also classified as normal or at-risk-for overweight according to CDC classification procedures. Findings: From 1999 to 2004, there was a significant increase in the average BMI of first graders, 15.8 plus or minus 2.2 kg/m[superscript 2] versus 16.8 plus or minus 2.2 kg/m[superscript 2], respectively (P less than 0.05). First grade boys had a progressive increase in BMI from 1999 to 2004 (15.6 plus or minus 2.2 kg/m[superscript 2] compared to 17.3 plus or minus 2.2 kg/m[superscript 2], respectively), but no change was evident for first grade girls. There was an approximate 4-fold increase in the percentage of rural first grade boys classified as at-risk-for overweight between 1999 and 2004. Conclusions: A progressive increase in the BMI and the significant increase in prevalence of at-risk-for overweight in rural first grade boys highlight the need for future gender and age group-specific investigations. Focus should be given to primary prevention programs targeting potentially vulnerable time periods when excess weight gain may be occurring.