Brain tumors in 4 dogs were treated with external beam, megavoltage radiation. X-ray computed tomography was used to localize and characterize brain tumors and to assess treatment response. Total radiation doses were 3,000 or 3,600 rad, given in 5 or 6 fractions over 14 to 19 days. Complete tumor regression, as determined from computed tomography scans, improvement in clinical signs, and reduction in medication, were documented in all irradiated dogs. The median survival time for irradiated dogs was 322 days, which was significantly (P less than 0.05) longer than the median survival time of 56 days in 8 dogs with brain tumors treated symptomatically. The one-year survival rate for the irradiated dogs, after correcting for deaths from intercurrent disease, was 100%. It was concluded that canine brain tumors may be treated effectively by use of megavoltage radiation.