We noticed almost routine disparate results in margin lengths when colorectal specimens are measured in vivo by the surgeon and in vitro by the pathologist. We studied 26 sigmoid and rectum specimens to document the amount of organ shrinkage after surgical removal and fixation. Each specimen had a 5.0-cm segment at each end of the specimen marked by serosal sutures before vascular devitalization. The segments were measured after the specimen sat unfixed for 10 to 20 minutes and after 12 to 18 hours of formalin fixation. The segments shrank to a median length of 3.0 cm (40% of the in vivo length) after 10 to 20 minutes and an additional 0.85 cm, to a median length of 2.15 cm, after fixation. Overall after fixation, the segments shrank 57% of the in vivo length. Approximately 70% of the shrinkage occurred during the first 10 to 20 minutes after removal, and the remaining 30% occurred after fixation. For optimal accuracy, margin distance must be obtained immediately after surgical removal. Once the specimen has been removed for several minutes, the difference between unfixed and fixed margin lengths is 30%. A correction factor of approximately 2x should be applied when interpreting the margin length.