128 acromegaly patients were followed up for 1-23 years (mean 5.9 +/- 5.0 years) after external pituitary irradiation; among them 104 patients received fractional irradiation with an ultimate total dose of 4000-5000 rad. The remaining patients received a total dose of either < 4000 rad or > 5000 rad. Human growth hormone (hGH) level was measured after an oral load of 100g glucose to evaluate the effect of irradiation. After external pituitary irradiation, the serum level of hGH decreased progressively. The decrease was more rapid in the first three years with a decrement of 35.1% per year. 36.2% of all the patients has been judged as clinically inactive with low serum hGH level of < or = 5 ng/ml. 62.1% of the patients was considered as slightly active with hGH level < or = 10 ng/ml in the 15th year after therapy. The effect of irradiation was not influenced by the age of the patients, the course of acromegaly, the size of sella turcica and operation prior to irradiation. If the total dose of irradiation is lower than 4000 rad, the effect will not be good.