Radiotherapy (RT) has long been used in the treatment of acromegaly, but confusion regarding the definition of biochemical cure has hampered interpretation of previous reports on the outcome of this treatment. In the present study we present additional data using the currently accepted criteria of biochemical cure in a large group of patients followed up by our department. Forty-six acromegalic patients were treated with external beam megavoltage RT and followed up for a mean of 7.6 years (range 2-22 years). Only four patients had had previous surgical treatment by either transsphenoidal or transfrontal routes. Following RT, mean basal GH levels decreased from 30.9 ng/ml (5-96 ng/ml) to 11.5 ng/ml (1-36 ng/ml) at 10 years of follow up with a further fall to 6.1 ng/ml (1-29 ng/ml) in those patients followed up for more than 10 years. As a result, although mean GH levels of less than 5 ng/ml were achieved in 9/28 (30.1%) at 5 years, 6/19 (31.6%) at 10 years, and in 6/11 (54.5%) of those patients followed up for more than 10 years post-RT, only 0/28 (0%), 7/28 (25%), 4/19 (21%) and 1/11 (1%) achieved GH levels of <2.5 ng/ml at 2, 5. 10 and >10 years following RT. Thus, in the whole series only 10/48 (20.8%) patients showed a decrease of GH level to less than 2.5 ng/ml at their latest follow up. Hypopituitarism as a result of RT was only infrequently observed in this series; gonadal deficiency developed in 12 (26.6%) patients, thyrotrophin (TSH) deficiency in 3 (6.6%) and adrenocorticotrophin deficiency in 2 (4.4%). In conclusion, megavoltage RT is an effective treatment for the control of GH hypersecretion in acromegaly, with a continuing lowering effect for several years following RT but seldom leads to safe GH levels.