BACKGROUND: Surgical treatment for perianal fistulas requires balancing fistula closure with the risk of complications such as incontinence. The advancement flap (AF) is a widely used sphincter-sparing technique, yet it appears to offer only marginally better outcomes compared to alternative techniques, with a notable incontinence rate. This study aimed to evaluate the success rate and long-term functional outcomes of AF at our tertiary referral center. METHODS: This retrospective cohort study analyzed prospectively collected data from electronic medical records and questionnaires distributed in December 2023. Patients aged 18 or older with primary or recurrent perianal fistulas treated with AF between 2013 and 2023 were included. Fistulas of non-cryptoglandular origin and rectovaginal fistulas were excluded. The primary outcome was fecal incontinence. Secondary outcomes included disease burden, fistula closure, and risk factors for recurrence. RESULTS: Eighty-one patients were included; 37 (46%) were women, mean age was 45 years, and 93% had a complex fistula. The median follow-up was 27 months (IQR 15.5-64). Before AF, 36% reported some degree of incontinence, increasing to 80% at long-term follow-up after AF. Specifically, 20 out of 26 (77%) preoperative fully continent patients reported incontinence issues at long-term follow-up. Fistula disease impact on daily life was higher for those who failed AF repair. Primary fistula closure was achieved in 35 patients (43%). No risk factors for AF failure could be identified. CONCLUSIONS: Advancement flap repair of perianal fistulas is challenging and can lead to fecal incontinence, so thorough preoperative counseling, consistent long-term follow-up, and further research comparing alternative sphincter-sparing techniques are warranted.