Background <p>Complete cytoreductive surgery (CRS) is critical for cure in peritoneal metastases (PM) from colorectal cancer (CRC) or ovarian cancer. This study prospectively evaluated whether an immediate second look improved detection of residual malignant nodules after CRS was deemed complete.</p> Methods <p>Eligible patients had histologically confirmed PM from CRC or ovarian cancer and were undergoing CC0 CRS performed by a senior surgeon. Once the procedure was complete, a second senior, blinded surgeon immediately conducted a full peritoneal re-exploration. Additional suspicious nodules were resected and sent for pathology analysis.</p> Results <p>From November 2021 to October 2024, a total of 172 patients underwent CC0 CRS for PM from CRC or ovarian cancer at one comprehensive cancer center, and 59 patients were enrolled. Most exclusions were for organizational reasons. PM originated from CRC in 35 patient and ovarian cancer in 24. The median surgical Peritoneal Cancer Index was 6 (interquartile range 4–14.5). The second look resulted in resection of additional suspected nodules in 88.1% of patients, and malignancy was pathologically confirmed in 47.4%. The proportion of patients in whom malignant nodules were missed during the initial inspection ranged from 25 to 80%. Surgeons demonstrated a high level of attention on cognitive tests conducted before and after the CRS. Errors and omissions remained rare but were consistently observed throughout the test.</p> Conclusion <p>Nearly half of patients had residual malignant disease after CRS initially judged as complete. These findings underscore perceptual limitations in intraoperative detection and support the utility of intraoperative systematic second look to enhance surgical thoroughness.η</p>