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Three-dimensional (3-D) endosonography is a new method of staging anal carcinoma that has not yet been validated in comparison with two-dimensional (2-D) endosonography, the latter using only a single scan plane. The aim of this study was to investigate the differences between the two endosonographic techniques.Thirty patients with an endosonographically detectable anal tumor were examined with a 10 MHz rotating endoprobe. Cross-sectional images of the anal sphincters were stored on a 3-D system during retraction of the endoprobe through the anal canal. Afterwards, any projection could be reconstructed. Cross-sectional images (2-D) were compared with reconstructed projections (3-D) according to five parameters concerning tumor spread and presence of regional lymph nodes. In this study, a scale of 0 to 5 points on critical issues was used; ideally, the results should be identical in 2-D and 3-D endosonography.The 3-D method detected a median of 5 diagnostic findings, compared with a median of 4 findings with the 2-D method (P= 0.001). In eight patients the lateral tumor margin was visualized only by 3-D endosonography. The median number of lymph nodes visualized in 3-D was 1 (range, 0-13), in 2-D the median number was 0 (range, 0-6),P= 0.002.Use of 3-D endosonography in patients with anal carcinoma improves detection of perirectal lymph nodes and may improve that of tumor invasion, compared with 2-D endosonography. This may affect local tumor staging and thus planning of treatment. A study with histopathologic correlation is needed to verify this endosonographic study.