Peritumoral edema is one of the most serious complications of intracranial neoplasms. However, the exact pathogenesis of peritumoral edema is still unknown. To explore the role of macrophages and LTs in brain tumors, we examined macrophage infiltrates in the brain tumors by immunohistological method using anti-Leu M3 monoclonal antibody and measured tissue LTC4 and LTD4 by HPLC with ODS column and UV spectrophotometer in the wavelength of 280 nm. Nine of 14 gliomas, 10 of 14 meningiomas, and all six metastatic brain tumors had macrophage infiltrates. The correlation was good between macrophage infiltration and peritumoral edema detected on CT. In many cases with peritumoral edema, LTC4 was detected. In most cases without peritumoral edema and many cases with preoperative steroid treatment, LTC4 was not detected. These data suggest that macrophages and LTs may play an important role in the formation of peritumoral edema.