18 patients with adult flexible flatfoot deformity scheduled to undergo surgical treatment were randomized to receive either lateral column lengthening or subtalar arthroereisis. The purpose of this study was to compare the efficacy of the two procedures in terms of radiological and clinical results. Radiographs, AOFAS Midfoot and Short-Form-36 scores were assessed over at least 18 months. The findings of this study indicate that subtalar arthroereisis is comparable to lateral column lengthening as no relevant radiological or functional differences were observed between the treatments at the end of the study period.