Peritoneal washing cytology during surgery was done in 745 patients with colorectal cancer. The positive washing cytology rate was 49/745 (6.6%). The peritoneal recurrence rates were 12/22 (54.5%) and 8/682 (1.3%) among patients with positive and negative peritoneal washing, respectively (p < 0.0001). The 5-year survival rate is 89.4% of the patients with positive cytology and 38.2% with negative cytology. The patients with positive cytology have a significantly lower survival rate than the negative one (p < 0.0001). Eleven patients of the positive cytology received intraperitoneal administration of MMC. Peritoneal dissemination occurred in 3/11 (27.3%) of the MMC treated group and 9/11 (81.8%) in the untreated group (p = 0.030). Our results indicated that intraperitoneal administration of MMC was an effective method of preventing peritoneal dissemination after resection of colorectal cancer.