Transrectal ultrasound (TRUS) is widely accepted as an accurate and effective technique for staging of rectal cancer. The decision to administer pre-operative chemotherapy/radiation therapy is often made based on the results of the TRUS. The greatest limitation of TRUS is its operator-dependence. Three-dimensional TRUS (3-D TRUS) is a new modality that provides all of the information of traditional TRUS as well as the ability to provide three dimensional views of the tumor. This provides greater detail regarding the configuration of the tumor as well as the ability to calculate tumor volume. The results of 3-D TRUS are interpreted after the study and thus, operator-dependence is reduced. There are no currently accepted techniques that allow for objective and accurate assessment of a rectal tumor's response to pre-operative treatment short of post-operative pathologic evaluation. Our objectives are to evaluate the ability of 3-D TRUS to quantify the tumor's response to pre-operative radiation therapy and/or chemotherapy and to correlate these findings with post-operative pathologic examination of the specimen. These objectives can be accomplished with 2 ultrasounds, one before treatment and one before surgery. All patients with biopsy-proven (with pathology confirmed at MSKCC) rectal cancer are eligible for the study. The size, volume and depth of penetration and lymph node involvement will all be assessed during both ultrasound examinations. Our findings will then be compared to those found at post-operative pathologic examination. These data will then be analyzed to determine if 3-D TRUS was accurate in predicting the tumor's response to preoperative treatment.