Peritoneal involvement is an important adverse prognostic factor in colorectal cancer (CRC) and determines a shift in the pathologic tumor node metastasis stage. Because peritoneal involvement is difficult to identify, use of special stains highlighting the peritoneal elastic lamina and mesothelial surface has been proposed. This study aims to determine whether use of elastic stain or CK7 immunohistochemistry on a single tissue section can refine the level of tumor invasion and determine whether restaging based on this assessment has prognostic significance in pT3N0 CRCs. Elastic stains were applied to 1 block per case from 244 consecutively resected pT3N0M0 CRCs. CK7 was evaluated in a 169-case subset. The elastic lamina was identified in only 101 cases (41%). Of those, 60 cases (24.6%) displayed elastic lamina invasion (ELI). This finding was associated with significantly worse (P<0.001) disease-free survival (DFS) (5-y DFS=60%) and significantly worse (P=0.01) overall survival (OS) (5-y OS=66.7%) compared with patients with no ELI (5-y DFS=87.8%, OS=92.7%) and those for whom no elastic lamina was identified (5-y DFS=82.5%, OS=86.0%). CK7 staining highlighted mesothelial cells in only 27 of 169 cases tested and helped demonstrate serosal invasion in only 5 cases (3%). In summary, the use of a single elastic stain is a useful and inexpensive method to demonstrate peritoneal involvement by tumor and should be considered for routine use in all pT3N0 CRCs. As tumors with ELI have an adverse prognosis, we propose that they should be upstaged compared with pT3N0 tumors without ELI.