Exfoliative cytology of peritoneal lavage was performed at the time of laparotomy for 458 patients with gastric cancer. Positive cytologic specimens were obtained from 28 per cent of all cases and from 16 per cent of patients with curative surgery. Peritoneal cytology has revealed itself to be a good predictor of prognosis and an indicator for adjuvant therapies. In the group treated with curative surgery, the five year survival rate was 86 per cent for patients with intact serosa and negative cytologic specimens, and 52 per cent for patients with involved serosa and negative cytology. None of the patients who had involved serosa and positive cytologic specimens survived more than five years. A slight benefit of adjuvant chemotherapy was observed in the patients who had curative surgery with negative cytology. Long term immunochemotherapy, including repeated intraperitoneal administration of mitomycin-C, improved the lifespan of patients with non-curative surgery.