Regression curves and local control rates of brain metastases after gamma knife treatment were evaluated to investigate differences in tumor response to radiation. A total of 203 metastases were serially evaluated using contrast-enhanced MRI (or computed tomography) at 1, 2, 3, 4.5 and 6 months after a 20-Gy dose. Differences were evaluated in regression curves and control rates between tumors ≥10 mm and tumors <10 mm in mean diameter, among three major histological subtypes of lung cancer, among adenocarcinomas of the lung, breast and colorectum, and between tumors in patients with above and below median hemoglobin levels. Smaller tumors shrank faster and yielded better control rates than larger tumors. Metastases from small cell and squamous cell carcinomas of the lung shrank faster than those from lung adenocarcinoma, but 6-month control rates were not different. Breast adenocarcinomas tended to shrink faster than lung adenocarcinomas, but the control rates were not different among adenocarcinomas of the lung, breast and colorectum. Tumors in patients with higher hemoglobin levels tended to shrink faster but the control rates were not different. Small cell and squamous cell carcinomas of the lung regress more rapidly than adenocarcinomas, although local control rates might not differ significantly.