Recent criteria for cure of acromegaly require normalization of the insulin-like growth factor (IGF-1) level. A retrospective study was conducted to assess postoperative sequential changes in the blood IGF-1 level and to determine the appropriate timing for endocrinologic assessment of the effect of surgery. Blood IGF-1 levels were measured at least 3 times (4.9 ± 2.0, mean ± SD) during the first postoperative year in 36 acromegalic patients whose glucose tolerance test results, obtained 3 months after surgery, showed the growth hormone level to be suppressed to under 1 ng/mL. Postoperative IGF-1 parameters, i.e. the actual IGF level and %IGF-1 compared to the nadir during the first postoperative year, rapidly decreased during the first 2 postoperative weeks and then slowly declined over the next 2 weeks. They reached a plateau (stable nadir) during the 2nd postoperative month. Assessment of the postoperative endocrinologic status should be delayed at least one month after surgery in acromegalic patients.