The purpose of the present study was to assess the size and configuration of the perirectal fatty tissues using magnetic resonance imaging, including the volume occupied by the rectum itself, and to establish a simple method by which such analysis could be derived. Included in the study were 25 consecutive patients without any large pelvic tumor (diameter of potential pelvic tumor less than 3 cm in any plane) referred for high-resolution pelvic MR imaging. The volume and cross-sectional parameters based on the amount of mesorectum to different sides of the rectum, and the total area occupied, including the rectum, were retrospectively measured using a transaxial three-dimensional T1-weighted gradient-echo sequence. The mesorectum, including the rectum within, occupied an axial area ranging from 320 to 5992 mm(2), and a total volume of 54-323 ml. There was a good correlation between anteroposterior diameter of the perirectal fat at 4 cm below S1-2 and the left-to-right diameter 7 cm below S1-2, and the total volume. Furthermore, the form of mesorectal tissue differed significantly between male and female subjects. In male subjects, measurements in the anteroposterior dimension accurately reflected the volume of mesorectal tissue, while in women, assessment of both the anteroposterior and the size parameters of the mesorectum from the left to right were required for the best evaluation of the volume of mesorectal tissue. The amount of fat posterior to the rectum was significantly more in men than in women, with or without consideration of length of the pelvis. Finally, the contour of the mesorectal fascia was subject to impression by other nearby visceral organs. There is a great individual variation in the amount of mesorectal fat, and in morphometric parameters between the two sexes. The morphological variations of the mesorectum can be assessed by magnetic resonance imaging using a formula based on two simple measurements of the anteroposterior and left-to-right dimensions.