There are no non-invasive methods to assess the real tumor size in rectal cancer prior to surgery, especially following radio/chemotherapy. Magnetic resonance imaging is gaining increasing acceptance as the primary modality at many centers for evaluation of pelvic malignancies including rectal cancers. The aim of this study was to evaluate if the tumor size as assessed by stereological or metric means on MRI correlates to the corresponding pathologic findings. To our knowledge, no such previous work has been reported in the literature. From the Cancer Register Center, 18 patients in the age range of 39-90 years with rectal cancer who had complete preoperative MR with subsequent giant section pathological examinations of the resected bowel were included. The tumor size was measured on MR and histopathologic specimen using both a stereologic and a metric mode. The measured parameters included the maximum transverse area occupied by the tumor, thickness, width, and the length of tumor and the volume of the tumor measured in two different fashions by the product of area and length (al) or the product of thickness, width, and length (twl). The depth of tumor infiltration (T) and presence of local lymph node metastases (N) were also separately evaluated on the histopathologic specimen. There were 1, 4, 12, and 1 patients with tumor stages T1, T2, T3, and T4, respectively. The mean thickness, width, length, area, and volumes, al and twl, were 1.62, 2.8, and 4.78 cm, and 4.72 cm2, 26.29 cm3, and 20.07 cm3, respectively. Regression curves were drawn for above-mentioned parameters. They showed some correlation with square correlation coefficient measuring between 0.38 and 0.82. The best correlation was seen for area (0.75) and volume measured by the product of area and length of the tumor (0.82). With the formula proposed from this material, we assume that rectal tumors can be measured on MR images using a metric model, especially area and the volume (the product of area and length), and then extrapolated to what we would expect from pathology, hence providing us with a tool where we could measure tumor response after neoadjuvant therapy.