Understanding the mechanisms by which growth hormone (GH) interacts with its receptor has led to the design of compounds that function as GH receptor antagonists. One such compound has been conjugated to polyethylene glycol (PEG) to produce a drug, pegvisomant, which has been extensively investigated as a treatment for acromegaly. It was recently approved for clinical use in the US and will shortly be available on prescription in Europe. Studies have shown that the drug is able to normalize circulating levels of insulin-like growth factor-1 (IGF-1), the principal mediator of GH action, in 97% of patients with active acromegaly, as well as improve the symptoms and signs associated with GH excess. Serum IGF-1 levels have been used as the chief marker of efficacy of treatment with pegvisomant. The drug is able to achieve biochemical control in patients wholly or partially resistant to somatostatin analogs. Preliminary data suggests that pegvisomant may be a particularly suitable choice of medical therapy for patients with acromegaly and coexistent diabetes mellitus.