Background: Although various prognostic factors in patients with colorectal cancer has been reported, the usefulness of intraoperative lavage cytology in patients with colorectal cancer is controversial. The aim of this study was to clarify the usefulness of intraoperative lavage cytology in patients with curative resection for pSage II-III colorectal cancer in a prospective multicenter study. Methods: The 20 member hospitals of the Japanese Society for the Cancer of the Colon and Rectum prospectively registered the patients diagnosed as stage II or III colorectal cancer preoperatively between 2013 and 2017. Among these patients, pStage II-III patients went through analysis. Lavage cytology was performed twice during surgery. The first procedure was performed right after laparotomy, and the second was performed right after specimen retrieval. The primary endpoint of this study was an effect of lavage cytology on 5-year relapse-free survival (RFS) in patients with pStage II-III colorectal cancer. The secondary endpoint was an effect of lavage cytology on 5-year overall survival (OS) and peritoneal recurrence in patients with pStage II-III colorectal cancer. Results: A total of 1378 patients were eligible and went through analysis. The number of patients with pStage II and III colorectal cancer were 670 and 708, respectively. Among 1378 patients, 54 (3.9%) had positive cytology. The median follow-up period of the entire cohort was 5.3 years. In pStage II patients, the 5-year RFS rate with positive and negative cytology was 61.1% and 81.6%, respectively (P = 0.023). The 5-year OS rate of pStage II patients with positive and negative cytology was 67.1% and 91.7%, respectively (P = 0.0083). However, there was no difference in RFS and OS between patients with positive and negative cytology in pStage III patients. Thirty-three patients had peritoneal recurrence. The peritoneal recurrence rate was 11.8% and 1.5% in pStage II patients with positive and negative lavage cytology, respectively (P = 0.032). That was 10.5% and 2.5% in pStage III patients with positive and negative lavage cytology, respectively (P = 0.022). In total, 11.1% of patients with positive lavage cytology had peritoneal recurrence in this cohort. Conclusions: The pStage II colorectal cancer patients with negative cytology had better RFS and OS compared to those with positive cytology. Peritoneal lavage cytology was useful in predicting peritoneal recurrence after curative resection for pStage II-III colorectal cancer. Clinical trial information: UMIN000026070.