OBJECTIVE: The purpose of this study was to determine whether the flat feet displayed by young obese and overweight children are attributable to the presence of a thicker midfoot plantar fat pad or a lowering of the longitudinal arch relative to that in non-overweight children. RESEARCH METHODS AND PROCEDURES: Foot anthropometry, an arch index derived from plantar footprints, and midfoot plantar fat pad thickness measured by ultrasound were obtained for 19 overweight/obese preschool children (mean age, 4.3 +/- 0.9 years; mean height, 1.07 +/- 0.1 m; mean BMI, 18.6 +/- 1.2 kg/m(2)) and 19 non-overweight children matched for age, height, and sex (mean age, 4.3 +/- 0.7 years; mean height, 1.05 +/- 0.1 m; mean BMI, 15.7 +/- 0.7 kg/m(2)). RESULTS: Independent t tests revealed no significant between-subject group differences (p = 0.39) in the thickness of the midfoot plantar fat pad. However, the overweight/obese children had a significantly lower plantar arch height (0.9 +/- 0.3 cm) than their non-overweight counterparts (1.1 +/- 0.2 cm; p = 0.04). DISCUSSION: The lower plantar arch height found in the overweight/obese children suggests that the flatter feet characteristic of overweight/obese preschool children may be caused by structural changes in their foot anatomy. It is postulated that these structural changes, which may adversely affect the functional capacity of the medial longitudinal arch, might be exacerbated if excess weight bearing continues throughout childhood and into adulthood.