BACKGROUND: This study was performed to evaluate the frequency and clinical characteristics of patients with active acromegaly and who show discordance of the growth hormone (GH) level and the insulin-like growth factor-I (IGF-I) level. METHODS: We reviewed the medical records of the patients who were diagnosed with acromegaly between 01/01/1995 and 6/30/2007 at Seoul National University Hospital. We selected only the patients whose basal GH and IGF-I levels were available. We investigated the pre- and post-operative clinical characteristics, as well as the blood concentrations of GH and IGF-I. The concordance rate between the two hormones was examined. The patients were considered to have active disease on the basis of their IGF-I levels above the normal range, after adjustment for age and gender, and their mean basal GH value was > or = 2.5 microgram/L. The hormone levels and the clinical parameters were compared between the hormone concordant and discordant groups. RESULTS: We reviewed the preoperative records of 103 acromegalic patients, and these patients met the above-mentioned criteria. 53 postoperative patients who were not cured by operation were monitored without them receiving radiation or medical therapy. Both the basal GH and IGF-I levels were above normal in 103 patients preoperatively, and the discordant rate was 0% (0/103 cases). Postoperatively, the discordant rate between the two hormones was increased to 30.2% (16/53 cases). Age, gender, body mass index and tumor size were insignificantly different between the concordant and discordant groups. However, postoperative residual tumors were less frequently observed in the discordant group (P = 0.006). CONCLUSION: For the patients with acromegaly, unlike the 0% discordance preoperatively, 30.2% of patients showed a discrepancy between their GH and IGF-I levels postoperatively. The patients who had hormonal discrepancy were less likely to have residual tumors after operation. Considering the frequency of this hormonal discrepancy, both hormone levels should be measured to evaluate the disease activity after treatment. Further, oral glucose tolerance testing should be performed and especially for the patients with an increased GH level, but who have a normal IGF-I concentration.