OBJECTIVE Sixty‐one of 83 patients with acromegaly treated between 1969 and 1993 were analysed retrospectively to clarify which early postoperative factors were significant predictors of a successful long‐term outcome and which preoperative factors significantly influenced the early postoperative results.PATIENTS Of the 61 patients, 30 were operated on before 1987 and 31 afterwards. A successful long‐term surgical outcome was defined as a long‐term mean basal GH level<6 mU/l (comparable to<3 μg/l), a normal IGF‐I level, and normal GH dynamics.RESULTS Overall, 59% of patients (37% before 1987 and 81% after) had an early postoperative mean basal GH level<6 mU/l, and 56% (29% before 1987 and 77% after) met all three of the specified criteria for a successful long‐term surgical outcome. Statistical analysis confirmed that GH dynamics and postoperative mean basal GH level<6 mU/l were significant predictors of the long‐term surgical outcome, whereas the postoperative IGF‐I level alone was not. On the other hand, abnormal preoperative GH dynamics were normalized in all patients with a postoperative mean basal level<6 mU/l. In addition, there were no patients showing an unsuccessful long‐term outcome in those associated with both the early postoperative mean basal GH level<6 mU/l and normalization of the IGF‐I level. Therefore, measurement of the early postoperative mean basal GH level and the IGF‐I level may be an economical and simple guide to predict the long‐term surgical outcome. Moreover, multivariate analysis indicated that cavernous sinus invasion was an independent significant factor influencing the early postoperative outcome.CONCLUSIONS Successful long‐term surgical outcome may be predicted if early postoperative mean basal GH level is reduced to<6 mU/l (<3 μg/l) and IGF‐I level becomes normal. This study also confirms that early diagnosis and treatment by an experienced endocrinologist and neurosurgeon can improve the operativ