Since 1988, we have performed intraoperative peritoneal washing cytology (CY) of the Douglas pouch in 1,255 patients (pts) with gastric cancer to diagnose latent peritoneal dissemination. Patients were classified as follows: 914 pts in P0, CY0; 100 pts in P0, CY1; 90 pts in P1; 53 pts in P2; and 98 pts in P3. Usually Papanicolaou's staining method was used and immunocytochemical staining of CEA and MOC-31 was carried out to make a definitive diagnosis. Prognostic factors were determined in 638 pts without liver metastasis by multivariate analysis. Curability, depth of tumor invasion, lymph node metastases, histological type and CY were independent prognostic factors. The survival of P0, CY1 pts who had undergone curative surgery was better than those with non-curative surgery. In conclusion, CY can serve as an independent predictor of prognosis, so immunocytochemical staining is important. P0, CY1 pts without residual cancer should be treated with multimodal therapy.