Active Phase: The active phase will last six weeks and include a clinic visit on the first day of the phase, laboratory testing between day 4 and 7, and telephone contact halfway through the phase. At the clinic visit, patients will receive their study medication (allopurinol or placebo) in a bottle prepared by the investigational pharmacy. Subjects on allopurinol will receive 10 mg/kg divided bid (maximum of 400mg). Laboratory tests will be performed 4 to 7 days after starting the medication to screen for hepatic or bone marrow toxicity (AST, ALT, CBC) and serum uric acid. Evidence for toxicity will cause un-blinding and withdrawal from the study. The families will also be instructed to continue the daily blood pressure log started in the screening phase. Twenty-four-hour ABPM and end of phase laboratory tests including, hemoglobin, serum uric acid, erythropoetin, ADMA, MCP-1 and urinary nitrates will be performed at the conclusion of the active phase, prior to discontinuing the study medication for the washout phase.