The aim of surgical treatment of fistula-in-ano is to eradicate the suppurative process permanently without compromising faecal continence. The appearance of a recurrence of fistula-in-ano is often due to non-identification of the internal opening by the surgeon, and to the presence of complex fistulae. We evaluated the clinical course of 214 patients in a randomised. controlled trial with respect to the recurrence rate of anal fistula with or without preoperative endoanal ultrasonography. In this study we demonstrate that endoanal ultrasonography was the most accurate diagnostic modality for detecting internal opening and complex fistulae. Therefore, endoanal ultrasonography is reliable and useful in the preoperative assessment of anal fistula, and particularly for decreasing the recurrence rate of this disease.