Arachidonic acid is stored in the cell membrane and released when the cell is activated by appropriate stimuli. It is the substrate for prostaglandins. Both experimental and human tumors often synthesize high levels of prostaglandins, most notably prostaglandin E2 (PGE2). Some experiments suggest that these compounds increase tumor growth through their actions on host immunocytes. In this study, 22 patients with various brain tumors and 12 control brain tissues were studied. PGE2 levels in tissue samples were measured by ELISA. Arachidonic acid levels in the plasma membrane of tissue samples were analyzed by capillary gas chromatography. The levels of PGE2 were significantly higher in gliomas (n = 10) and meningiomas (n = 7) compared with control tissues (P = 0.000 and P = 0.000, respectively). Also, PGE2 levels in meningiomas were significantly higher than in gliomas (P = 0.000). Arachidonic acid levels in the plasma membrane of gliomas (n = 9) and meningiomas (n = 6) were significantly higher than in the control tissues (P = 0.000 and P = 0.000, respectively). These results suggest that the increased production of PGE2 may suppress the immune system and play an important role in tumor growth.