Thirty-three patients with acromegaly were treated with external pituitary irradiation between 1976 and 1986 They were followed up at 1.3. and 5 years after treatment. The mean follow-up was 3.67 + 1.55 years. The plasma growth hormone levels were reached only in some patients during a prolonged period of time. Plasma growth hormone concentration was 10 ng/ml or less in 47 percent and was 5 ng/ml or less in 24 percent of patients at 5 years. The time period that is needed for remission was to be 3.50 + 1.41 years. The larger tumor size, high plasma growth hormone concentrations and hyperprolactinemia seem to affect the prognosis adversely.