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This study was undertaken to evaluate the accuracy of endoanal ultrasonography for preoperative assessment of anal fistula, with special reference to the difference between acute and chronic fistula. The subjects comprised 401 patients treated for acute or chronic anorectal sepsis of cryptoglandular origin during the period January through December 2005. All patients underwent physical examination and endoanal ultrasonography. Agreement between the physical and endosonographic findings and the definitive surgical findings were evaluated with special reference to classification of the primary tract and horseshoe extension and localization of the internal opening. The difference in accuracy of endosonographic assessment between acute and chronic fistula was also evaluated. The accuracy of endoanal ultrasonography was significantly higher than that of physical examination in detecting the primary tract (88.8% vs. 85.0%, p=0.0287) and horseshoe extension (85.7% vs. 58.7%, p<0.0001) and in localizing the internal opening (85.5% vs. 69.1%, p<0.0001). Furthermore, localization of the internal opening by endosonography was significantly more accurate in chronic fistula than in acute fistula (89.5 % vs. 76.8%, p<0.0001), although the accuracy in detecting the primary tract and horseshoe extension was not significantly different. Endoanal ultrasonography is reliable and useful for preoperative assessment of anal fistula, particularly for detecting horseshoe extension and localizing the internal opening. Endosonographic assessment provides clearer depiction of the internal opening during periods of quiescence than during the period of abscess formation. For patients with acute anorectal sepsis, initial surgical drainage and subsequent fistula surgery, rather than one-stage fistula surgery, may be advisable to avoid misidentification of the internal opening. [ABSTRACT FROM AUTHOR]