BACKGROUND: Several studies have found positive correlation between flatfeet and increased body mass in children. One study, utilizing a differing method of foot posture assessment, found the inverse. The purpose of this study was to further explore the relationship between children's foot posture and body mass, utilizing the foot posture index in a large study population, as opposed to the footprint based measures of most previous studies. METHODS: Data for both foot posture index (FPI) and body mass index (BMI) for healthy children were acquired from five previous studies. The amalgamated dataset comprised observations for both BMI and FPI-6 in 728 children aged from three to 15 years. Three FPI-6 scores levels defined the range of flatfeet detected: FPI-6 ≥ +6; FPI-6 ≥ +8; FPI-6 ≥ +10. BMI cut-points were used to define overweight for each age group. RESULTS: In the study population of 728 children, flatfeet (FPI ≥ +6) were found in 290 (40 %) cases and non-flatfeet in 438 (60 %) cases. FPI ≥ +8 yielded flatfeet in 142 (20 %) cases and FPI ≥ +10 yielded flatfeet in 41 (5 %) cases. Whilst 272 (37 %) children were overweight, only 74 (10.1 %) of the overweight children had flatfeet (FPI ≥ +6), which diminished to 36 (4.9 %) at FPI ≥ +8, and 9 (1.2 %) at FPI ≥ +10. Significant and moderate correlation was found between BMI and age (r = 0.384, p < 0.01). Very weak, but significant, correlation was found between BMI and FPI (r = -0.077, p < 0.05). Significant mean differences between gender and BMI were found (t-test = 2.56, p < 0.05). There was strong correlation between FPI scores on left and right sides (r = 0.899, p < 0.01). CONCLUSIONS: This study found no association between increased body mass and flatfeet in children, a finding in contrast to that repeatedly concluded by many previous studies. Whilst properties of the FPI and BMI are limiting, these findings question the concern about children's increased body mass as a specific influence on (flatter) foot posture, and also the validity of footprint versus anatomically based foot posture measures.