CSF glutamine concentrations were studied in 12 patients with benign brain tumors (meningioma, craniopharyngioma, or osteofibroma), 12 patients with malignant brain tumors (astrocytoma, medulloblastoma, pinealoblastoma, or chondrosarcoma), 9 patients with noncerebral tumors, and a reference group of 24 patients. The mean +/- SD levels in the benign tumor group (424 +/- 124 microM) were significantly lower (p less than 0.0004) than those in the reference group (642 +/- 195 microM). There was no significant difference between the CSF glutamine concentrations in the malignant cerebral tumor group (643 +/- 210 microM) or noncerebral tumor group (599 +/- 127 microM) and those in the reference group. In patients with benign brain tumors there was indication of an inverse linear relationship between the logarithm of CSF glutamine concentration and tumor diameter.