A cross-sectional study of preadolescent boys was performed to evaluate whether grouping by BMI and PBF would produce similar results. Participants included fifty 8-to10-year old Caucasian boys. Height and weight were measured to calculate BMI and PBF was determined using air-displacement plethysmography. Boys were grouped based on their BMI into an average BMI group (BMI percentile between the 33rd and the 68th) and a high BMI group (BMI percentile ≥85th) and based on their PBF into a lower PBF group (PBF <21.8) and an upper PBF group (PBF >21.8). Independent samples t-tests were used to examine differences in age, weight, height, BMI, BMI percentile and PBF for boys by their group status. Correlation analysis was used to examine the relationship between boys’ BMI and PBF. A chi-square test was used to examine similarities in proportions for possible outcomes of groupings by BMI and PBF. Differences were found in weight, BMI, BMI percentile and PBF between the groups when they were divided by BMI and PBF. PBF was significantly correlated with BMI (r=0.74,p<.0001), and the two ways of grouping were not independent (χ2=13.52, p<.0001). Six of the 25 boys in the high BMI group were classified into the lower PBF group, and 6 of the 25 boys in the average BMI group were classified into the upper PBF group, indicating that 6 boys from each group were classified differently. BMI may be a useful screening tool to determine adiposity in groups, but it can be problematic when evaluating individual boys.