To elucidate the correlating factors and clinical significance of flexible flatfoot (FF) in preschool children of Taiwan, a cross-sectional study was conducted in a research laboratory equipped with a gait analysis facility. Altogether, 377 preschool children (201 boys, 176 girls), ranging in age from 2 to 6 years, were enrolled in this investigation. The results show that age, height, weight, foot progression angle, occurrence of physical knock-knee, and joint laxity score correlate with FF. Children with FF, compared with children without, performed physical tasks poorly and walked slowly, as determined by gait parameters. The FF should not simply be regarded as a problem of static alignment of the ankle and foot complex, but may be the consequence of a dynamic functional change of the lower extremity. Better understanding of the correlating factors and the clinical relevance of FF may prove helpful in deciding on the most appropriate treatment for a particular patient.