This study is a Phase IV, open-label, single-arm study to assess the safety and the necessity of dose adjustment after switching to FLOLAN injection prepared with the reformulated diluent in Japanese patients with PAH who are receiving higher doses of FLOLAN injection than in other countries. The objective is to evaluate the safety and tolerability of the thermostable formulation of FLOLAN injection (that is [i.e.], FLOLAN injection prepared with the reformulated diluent) when switched from the existing FLOLAN injection treatment (i.e., FLOLAN injection prepared with the currently marketed diluent). The study will include a screening visit, a run-in period of a maximum of 4 weeks with the existing FLOLAN treatment (i.e., FLOLAN injection prepared with the currently marketed diluent), a 4-week treatment period with the thermostable formulation of FLOLAN injection (i.e., FLOLAN injection prepared with the reformulated diluent) and a one-week follow-up visit. Adequate number of subjects will be enrolled in the study in order to have 10 subjects to complete assessments at 4 weeks, including at least 5 subjects as a subset of subjects who consent to undergo right heart catheterisation (RHC) over 24-hour and at Week 4. FLOLAN is a registered trademark of the GlaxoSmithKline [GSK] group of companies.