We measured the effect of the radiation enhancer metronidazole on patients with metastatic brain tumors undergoing radiation therapy (RT) in a randomized controlled study. Metronidazole was given in a dose of 6 g/m2 4 hr before each of the first 3 doses of RT to the whole-brain. A total of 3000 rad was delivered in 6 doses. Patients were followed by serial neurological examinations and CT scans. The metronidazole group did not differ from the control group when measured in terms of survival, clinical improvement or improvement on CT scan, whether measured 2 months or 6 months following treatment. Nausea and vomiting was a significant side effect, preventing about 10% of patients from completing the treatment. Metronidazole appears to have no substantial radiation enhancing effect on metastatic brain tumors.