Peritoneal seeding is frequently detected in patients with gastric cancer. The peritoneal cavity is a compartment in which the immunologic host-tumor interaction can occur. Here, we investigated the gene expression levels of cytokines, and compared these gene expressions with the progression of gastric cancer. Total RNA was extracted from 50 ml of peritoneal washings from 78 patients with gastric cancer and 11 noncancerous patients. The gene expression levels of transforming growth factor-beta (TGF-beta), interleukin (IL)-2, IL-6 and IL-12 were analyzed by real-time reverse transcription-polymerase chain reaction. The gene expression levels of TGF-beta and IL-12 in 16 patients with peritoneal seeding (peritoneal metastasis or free cancer cells) did not differ from those in controls (n = 11) and stage I and II (n = 43) patients. However, the relative gene expression level of IL-2 in patients with peritoneal seeding (0.9) was lower than that in controls (1.4, p = 0.066) and stage I and II patients (1.4, p = 0.036). In contrast, the relative gene expression level of IL-6 in patients with peritoneal seeding (3.3) was higher than that in control (2.4, p = 0.064) and stage I and II patients (2.6, p = 0.065). Low IL-2 gene and high IL-6 gene expressions in the peritoneal cavity may correlate with cancer development in the peritoneal cavity in patients with gastric cancer.