Pegvisomant is a GH receptor antagonist and strong inhibitor of insulin-like growth factor I (IGF-I) production. The treatment goal for acromegaly is to normalize serum IGF-I levels and attenuate associated symptoms. The efficacy and safety of pegvisomant as treatment for acromegaly have been reported in Caucasians, but not in Japanese. Here we report the clinical experience of using pegvisomant in Japanese patients with acromegaly. The efficacy and safety data for pegvisomant from two open-labeled clinical studies in Japan, conducted from 2004 to 2007, were re-analyzed using the new Japanese age- and sex-matched normative ranges for IGF-I. Eighteen patients with active acromegaly were enrolled in an initial pivotal study, and 16 of them were moved to a long-term (max 168 weeks) extension study. The dose of pegvisomant in the extension study was adjusted to 10-30 mg per day according to IGF-I levels. IGF-I normalization was observed in 81.3% (13/16 patients) during the extension study. The mean percentage decrease from baseline in serum IGF-I level was 64.7% at the time of last observation. The clinical symptoms and overall health status were improved, and the ring size was reduced over time until Week 12 and maintained. For safety, no clinically significant changes were observed both in the pituitary tumor size and the anti-GH antibody level. Three subjects were withdrawn from the studies due to an abnormal elevation of liver enzymes which resolved after discontinuation. Pegvisomant demonstrated excellent clinical efficacy and was well tolerated in Japanese patients with acromegaly.