A total of 71 cases with primary brain tumors (44 cases) and metastatic brain tumors (30 cases) were entered into our clinical studies with MCNU and radiation therapy or MCNU alone. With regard to tumor reduction on CT scan, the response rates obtained for MCNU and radiation were 21.7% for malignant gliomas and 50.0% for metastatic brain tumors. With regard to improvement of neurological signs, the response rates obtained for MCNU and radiation were 62.9% for malignant gliomas and 71.4% for metastatic brain tumors. The response rates for MCNU were 5.8% for malignant gliomas and 46.1% for metastatic brain tumors. In the improvement of performance status, the response rates for MCNU and radiation were 51.8% for malignant gliomas and 64.2% for metastatic brain tumors. The response rates for MCNU were 46.1% for malignant gliomas and 30.7% for metastatic brain tumors. A minimal degree of hematological toxicity occurred but this gradually disappeared. These results suggested that MCNU has relatively effective antitumor activity against metastatic brain tumors and an enhanced effect with radiation against malignant gliomas.