The microscopic detection of free peritoneal tumour cells in peritoneal lavage fluid in gastric cancer patients is a useful predictor of peritoneal recurrence and poor prognosis. The aim of this study was to verify the prognostic significance of intraoperative peritoneal lavage cytology and its value as a predictor of peritoneal recurrence. We evaluated the presence of free peritoneal tumour cells with extemporary cytological examination in a series of 170 peritoneal washing samples from patients undergoing gastrectomy for gastric cancer over the period from January 1992 to June 2001. Twenty-eight patients (16%) had positive extemporary lavage cytology and there were no false-negatives as compared with the final examination. All patients with positive cytology presented serosal infiltration (T3/T4). Positive peritoneal lavage cytology was a predictor of poor prognosis and peritoneal recurrence: the 24 month survival rate was 17% for positive and 60% for negative cases (P = 0.003); in positive cases 71% of recurrences were located in the peritoneum. Intraoperative cytological examination of peritoneal washings can detect the presence of free malignant cells in the peritoneal cavity and can be used to select patients who may benefit from intraperitoneal chemotherapy.