Introduction: There is considerable debate as to the ideal procedure for the treatment of symptomatic flexible flatfoot (FFF) that is not followed by recurrence and minimizes complications. The purpose of the present study was to evaluate the results of calcaneal lengthening using Mosca's technique in adolescents with FFF in whom conservative treatment had been applied for more than one year but had failed. Patients and Methods: Calcaneal lengthening osteotomy was performed using Mosca's technique in 19 feet of 14 patients with ten males, four females and five bilateral cases. The mean age at surgery was 13.53 (range 11.5-16) years. All FFF patients were evaluated as idiopathic. Bilateral cases were operated on at two sessions with an average interval of 15.6 months (range 12 to 21). The American Orthopaedic Foot and Ankle Society (AOFAS) ankle-hind foot scale was used for clinical assessment, and radiographic assessment was based on six parameters on standard anteroposterior and lateral radiographs. Results: The mean follow-up period was 27.89 (range 18-44) months. The mean AOFAS score increased from 57.53 preoperatively to 96.32 postoperatively. All radiographic parameters significantly improved. Four patients had mild occasional pain. There was no nonunion or secondary subsidence of the arch. All Patients stated they were satisfied with the procedure. Conclusion: Correction of flexible flatfoot deformity with Mosca's lateral calcaneal lengthening was an effective and reproducible method to restore normal foot alignment and good function.