PURPOSE: Idiopathic flexible flatfoot (IFF) is a frequent condition in children. Patients refractory to conservative treatments may benefit from surgical procedures. The aim of this study was to evaluate clinical outcomes and sport activity levels in a large cohort of paediatric patients treated with calcaneo-stop (CS) for the symptomatic IFF. METHODS: A single-centre retrospective study was conducted using an institutional database that prospectively collected clinical outcomes of patients treated with CS for symptomatic IFF. The procedure included the implantation of a cancellous screw through the talus, which was subsequently removed after 2 years. A total of 644 feet (336 consecutive patients) followed up to a mean of 41.3 ± 6.7 months after implant removal were included. Foot pain and sport activity were assessed. RESULTS: A successful outcome, defined as the presence of a painless, corrected foot together with patient satisfaction, was obtained in 94% of the patients, while 35 feet were considered failed. Extracurricular sport participation was possible in most patients after CS screw implantation (55%) and fully recovered after implant removal (77%). No activity level increase was shown compared to baseline. The only factor that correlated significantly with the incidence of failures was the occurrence of adverse events ( CONCLUSIONS: CS procedure provided highly satisfactory clinical results at mid-term follow-up. Full sport activity level was recovered after screw removal and a 94% favourable outcome was obtained in terms of foot pain relief as well as patient and parents satisfaction. LEVEL OF EVIDENCE: Level IV, case series.