OBJECTIVE: To assess the accuracy of anal ultrasound (AUS) for anal fistulas, and the impact of routine pre-operative AUS on their surgical management. METHODS: Pre-operative AUS was performed in 38 consecutive patients with an anal fistula using a 10-MHz Brüel & Kjaer probe. All patients underwent subsequent examination under anaesthetic (EUA) with documentation of the anatomy of the fistula before the surgeon was shown the AUS results. Agreement between AUS and EUA findings and any modification to the surgical treatment was recorded. RESULTS: There was 84% agreement between AUS and EUA findings regarding presence and site of fistulas. One fistula not seen at AUS was found at EUA, and 5 fistulas seen on AUS were not demonstrated at EUA. AUS influenced the surgery undertaken in 9/24 (38%) patients; demonstrating occult sphincter defects (2 patients), reclassifying fistulas from low to higher fistulas (3 patients), deciding a surgical treatment open to doubt (2 patients) and helping identify an obscure fistula not initially found at EUA (2 patients). CONCLUSIONS: Accuracy of AUS in the assessment of anal fistulas is confirmed. Operative management is influenced in 38% of cases, usually towards more conservative treatment. We recommend the use of pre-operative AUS in the assessment of anal fistulas.