Context: Pegvisomant is a GH receptor antagonist. The ACROSTUDY is a global safety surveillance study of long-term treatment of acromegaly with pegvisomant. Objective: The objective of the study was to monitor long-term safety and treatment outcomes. Design: ACROSTUDY is open to all patients with acromegaly who are treated with pegvisomant. We report an interim analysis of data captured from 1288 subjects enrolled before a database freeze of December 31, 2009. Setting: This was a global noninterventional surveillance study. Main Outcome Measure(s): Long-term monitoring of safety, including central magnetic resonance imaging (MRI) reading and treatment outcomes, was measured. Results: Subjects (n = 1288) were treated with pegvisomant for a mean of 3.7 yr and followed up in ACROSTUDY for a mean of 2.1 yr. A total of 1147 adverse events (AE) were recorded in 477 subjects (37%), among which 192AEin 124subjects (9.6%) were considered to be related to pegvisomant. Serious AE were recorded in 159 subjects (12.3%), whereas pegvisomant-related Serious AE were recorded in 26 subjects (2%). No deaths (15 subjects; 1.2%) were attributed to pegvisomant use. The incidence of increase in pituitary tumor size in the subset with confirmed MRI increases on central reading represented 3.2% of the overall cohort with at least two available MRI (n = 936). Injection-site reactions were reported in 28 cases (2.2%). In 30 patients (2.5%), an elevated aspartate aminotransferase or alanine aminotransferase of more than 3 times the upper level of normality was reported. There were no reports of liver failure. After 5 yr of pegvisomant treatment, 63.2% of subjects had normal IGF-I levels at a mean dose of 18 mg/d. Conclusions: Data entered and evaluated in ACROSTUDY indicate that pegvisomant is an effective and safe medical treatment in patients with acromegaly. The reported low incidence of pituitary tumorsize increase, liver enzyme elevations, and lipodystrophy at the injection site are reassuring. Copyright © 2012 by The Endocrine Society.