OBJECTIVES: To define the true anatomic structure of the rectourethralis muscle. METHODS: Cross-sectional images of fresh tissues from the Visible Human Data set were reviewed. Three-dimensional computer reconstructions of the rectourethralis and surrounding structures were generated from these data using a high speed computer and imaging software. The structure of rectourethralis was confirmed by dissection of fresh cadavers. RESULTS: The rectourethralis arises deep within the substance of the smooth muscle of the rectal wall as two limbs of muscle which face posterolaterally. These limbs fuse into a single thick muscle which inserts into the perineal body. CONCLUSIONS: In contrast to all previous descriptions in the literature, the rectourethralis is a Y-shaped muscle which arises within the substance of the rectal wall deep to the outer longitudinal smooth muscle. This shape is consistent with the muscle's embryologic origin. Knowledge of the structure of the rectourethralis muscle will help urologists avoid rectal injuries during perineal approaches to the prostate.