Free cancer cells exfoliated from the cancer-invaded serosa contribute to peritoneal dissemination, the most frequent pattern of recurrence in gastric carcinoma patients. This study was designed to evaluate the prognostic relevance of such free cells in peritoneal washes detected by reverse transcriptase-polymerase chain reaction (RT-PCR) and cytology. RT-PCR analysis with primers specific for carcinoembryonic antigen and conventional cytologic examination by Papanicolaou staining were performed on peritoneal washes, collected at laparotomy from 148 gastric carcinoma patients. Prognostic analyses were performed with 1) death due to cancer recurrence and 2) peritoneal dissemination as endpoints. RT-PCR was found to be more sensitive than cytologic examination for detection of free cancer cells in the peritoneal washes, with a higher detection rate for each of the T categories in the tumor-node-metastasis (TNM) classification. Five patients with synchronous or recurrent peritoneal dissemination were found among 17 patients with positive RT-PCR and negative cytologic results. Both positive cytologic results and positive RT-PCR results had significant influences over the survival of patients with advanced gastric carcinomas (n = 75, p < .002). Detection of free cancer cells in peritoneal washes, most reliably by RT-PCR, is promising as a predictor of peritoneal dissemination in patients with gastric carcinoma.