OBJECTIVE: Although boys are frequently reported to have flatter feet than girls, there has been little systematic research to confirm or explain this structural difference between the genders. The objective of this study was to determine whether flat-footedness was moderated by gender in Australian preschool children and, if so, to determine the cause of this between-gender difference in structure of the plantar surface of the foot. METHODS: Foot anthropometry, Arch Index derived from plantar footprints, and midfoot plantar fat pad thickness measured by ultrasound were obtained for the left and right feet of 52 girls and 36 boys (mean age, 4.2 +/- 0.6 years). The children were recruited from 10 randomly selected preschools from the Illawarra region of New South Wales, Australia. RESULTS: In agreement with previous research, the preschool boys displayed significantly flatter feet than the girls (P < or = 0.04). Although there were no between-gender differences in structural foot dimensions, the boys had a significantly thicker midfoot fat pad than the girls by approximately 0.4 and 0.5 mm on both the right and left feet, respectively. CONCLUSIONS: The increased incidence of flat-footedness in boys compared with girls of the same age seems to be caused by a thicker plantar fat pad in the medial midfoot in boys. This suggests that the development of the medial longitudinal arch may be progressing at a slower rate in boys than in girls, and that intervention for a flexible flat foot, particularly for young boys, may be unnecessary.