Despite the increasing trends, reports on long-term follow-up are limited on transitioning from parenteral to oral treprostinil therapy in patients with pulmonary arterial hypertension (PAH). We investigated both the effectiveness of parenteral to oral treprostinil transition and the characteristics associated with transition failure over a duration of two years. The study included 37 Group I functional class I and II patients with PAH on combination therapy. Patients were excluded if cardiac index ≤2.2 L/min/m2, right atrial pressure ≥11 mmHg, or 6-min walk distance ≤250 m. Patients were categorized as successful (STransition) or unsuccessful (UTransition) transition based on clinical stability, or a parenteral comparator (CParenteral) if they remained on parenteral therapy (no transition). All patients underwent two right heart catheterizations, one at enrollment and a second post transition. Of 24 total transition patients, 46% were classified as UTransition. UTransition occurred on average 577 days post transition. Both UTransition and STransition had similar hemodynamics at diagnosis and treprostinil dose before and after transition. Before transition, the pulmonary vascular resistance (PVR) was significantly higher in the UTransition (6.7 ± 2 WU) vs. STransition group (3.5 ± 1.5 WU). At follow-up catheterization, the UTransition group demonstrated further increases in PVR, greater than the CParenteral group, without recovery despite “rescue” therapy in the UTransition group. A pre-transition PVR of 4.16 WU discriminated the UTransition from the STransition group. While a subset of PAH patients on combination therapy may be safely transitioned from parenteral to oral treprostinil, caution should be exercised in patients with elevated baseline PVR to avoid irreversible destabilization.