Tumor-associated macrophages(TAM)of the M2 phenotype(M2 macrophage)promote tumor proliferation and are associated with a poor prognosis in several tumors. We investigated the phenotype of macrophages in gastric cancer patients with peritoneal carcinomatosis(PC). Intraperitoneal macrophages were found more frequently in PC patients than in Stage I or II patients(control). CD163, an M2 macrophage surface marker, was detected in 71.7% of PC patients and 15.7% of controls. In addition, we examined whether macrophage markers were present in ascites caused by gastric cancer following intraperitoneal paclitaxel. The ratio of M2 macrophages decreased with every therapy. These findings indicate that intraperitoneal M2 macrophages may contribute to the progression of PC, and inhibition of M2 macrophages could be a promising treatment strategy.