The presence of free tumour cells at the mesothelial surface in patients with intraperitoneal colon cancer is an important prognostic factor and may alter the decision regarding adjuvant chemotherapy. In this prospective study of 72 patients we aimed to show the existence of malignant cells in the peritoneal cavity by scraping the serosa and by peritoneal lavage. Intraoperative peritoneal cytology was performed in 72 colon cancer patients who underwent curative surgery in our department between 1992 and 1995. Specimens were obtained by peritoneal lavage and by scraping the serosa overlying the major tumour mass and an area of normal serosa at least 10 cm from the primary site. Slides were stained with haemotoxylen-eosine stain. Statistical analysis was performed with chi-squared and Fisher's exact tests. In conclusion, peritoneal washing and serosal cytological study appeared to be a simple and reliable method to detect serosal involvement and the existence of malignant cells in the peritoneal cavity.