Objective: To evaluate use of pegvisomant, a growth hormone (GH) receptor antagonist, as monotherapy in ACROSTUDY, a global safety surveillance study set in 14 countries (373 sites). Methods: A descriptive analysis of safety, magnetic resonance imaging (MRI) reading, and treatment outcomes in 710 subjects who received at least 1 pegvisomant dose as monotherapy during and up to 5 years follow-up in ACROSTUDY. Results: Subjects received a mean of 5.4 years of pegvisomant and were followed in ACROSTUDY for a mean of 3.8 years. A total of 1,255 adverse events (AEs) were reported in 345 subjects (48.6%). Serious AEs (SAEs) were reported in 133 (18.7%) subjects, including 22 deaths, none of which were attributed to pegvisomant use. Of 670 (94%) subjects with at least 1 liver function test (LFT) reported in ACROSTUDY, 8 (1.2%) had reported increases in transaminases >3 times the upper limit of normal (ULN). No liver failure was reported. Based on central MRI reading, 12 of 542 subjects (2.2%) had a confirmed increase or increase/decrease in tumor size. Injection-site reactions were reported in 2.3%. At 5 years of therapy, insulin-like growth factor 1 (IGF-1) level was reported normal in 67.5% (mean dose 17.2 mg/day) and elevated in 29.9% (mean dose 19.8 mg/day). Subjects on 20 mg per day or more rose from 36% at 3 years to 41% at 5 years of therapy. Conclusions: ACROSTUDY data indicate that pegvisomant used as sole medical therapy is safe and effective for patients with acromegaly. The reported low incidence of pituitary tumor size increase and liver enzyme elevations are reassuring and support the positive benefit-risk of pegvisomant therapy.