Acromegaly is a rare, chronic, and debilitating disease, usually caused by a benign tumor on the pituitary gland, which leads to excessive production of growth hormone (GH). GH excess in turn causes overproduction of another hormone called insulin-like growth factor-1 (IGF-1). IGF-1 levels are currently the most widely accepted measure of disease activity.

In Canada, medical therapy with a type of medicine called "somatostatin analogues" (SSA), such as octreotide and lanreotide, is recommended for treatment of acromegaly. However, studies have shown that a significant number of patients who take SSA medications alone remain with elevated levels of IGF-1 in their blood.

Another medication that is used to treat acromegaly is pegvisomant (PEGV), and the investigators plan to study whether strict control of IGF-1, by adding or optimizing the use of PEGV, results in a significant health benefits to patients who still have modestly high levels of IGF-1 in their blood.

Inclusion Criteria:

• Adult patients (at least 18 years old) with confirmed acromegaly whose IGF-I levels are persistently but modestly (1.0 ULN < [IGF-1 serum level] < 1.5 ULN) elevated following medical therapy such as SSA, PEGV, cabergoline alone or in combination.

Exclusion Criteria:

1. Progressive or recent visual field loss or optic chiasmal compression, or pituitary tumors within 2mm from the chiasm. Patients whose visual field loss, optic chiasmal compression or pituitary tumor has been stable for at least a year will be eligible.
2. Cranial nerve palsies or intracranial hypertension requiring tumour decompression surgery
3. Clinically significant hepatic disease and/or elevated liver enzymes (ALT, AST > 3 x ULN)
4. Patients who have received pituitary surgery within one year prior to screening visit
5. Patients who have received radiation therapy within one year prior to screening visit
6. History of hypersensitivity to any components of Pegvisomant
7. Inability to fully comprehend the nature of the study or cooperate with study procedures
8. Pregnant / lactating women and subjects refusing to use adequate contraception to prevent pregnancy during the study.
9. Subjects unwilling or unable to self-administer medication on a daily basis
10. known or suspected alcohol / drug abuse
11. Severe acute or chronic medical or psychiatric condition or laboratory abnormality that could increase the risk associated with trial participation.