Context .- Peritoneal elastic lamina invasion (PELI) has been reported to be an important adverse prognostic factor in pT3 colorectal cancer (CRC). However, the data supporting this contention are limited. Objective .- To clarify the associations between PELI of pT3 CRC and prognostic significance, 139 consecutive surgical cases of pT3 CRC were examined. Design .- One hundred thirty-nine consecutive in-house surgical cases of pT3 CRC between 1993 and 2011 were examined. Thirty consecutive surgical cases of pT4a CRC resected during the same period were examined for comparison. Case selections were restricted to pT3 CRCs with the sections containing the deepest adenocarcinoma invasion partially or entirely covered with the peritoneum. Elastic staining was performed on one section containing the deepest tumor invasion partially or entirely covered with the peritoneum. The associations between the presence of PELI and clinicopathologic factors including prognosis of the patients were examined. Results .- Peritoneal elastic lamina invasion was identified in 23.0% (32 of 139) of the pT3 CRCs. PELI was associated with primary site (P = .006), lymph node metastasis (P < .001), lymphovascular invasion (P < .001), recurrence (P = .007), and patient's age (P = .002). The proportions of patients with a 4-year recurrence-free period in those with negative PELI, positive PELI, and pT4a tumor were 90.3%, 66.7%, and 28.9%, respectively (P < .001). Conclusions .- Elastic staining is useful to evaluate the serosal invasion of CRC. Positive PELI is a significant predictive factor for lymph node metastasis and recurrence-free survival in patients with pT3 CRC. This indicates that pT3 tumors with PELI should be treated like pT4a tumors.