 Patients meeting all inclusionexclusion criteria during the Screening Phase will enter the Baseline Phase, during which baseline exercise capacity, vital signs, and clinical signs and symptoms of the disease will be assessed. After confirmation of all inclusionexclusion criteria, patients will be assigned to study drug Remodulin or placebo and will enter the Treatment Phase. The Treatment Phase begins with a Dose Transition Period, during which patients will begin receiving subcutaneous study drug at a low dose determined by the patients current dose of Flolan. The study drug dose will be increased gradually while the Flolan dose is decreased gradually over a period of up to 14 days. The dose changes will continue until Flolan therapy has been discontinued and the patient is stable on study drug. 