The object of this study is to assess long-term safety, tolerability and efficacy of bosentan in patients with pulmonary hypertension (PH) associated with sickle cell disease (SCD). The study population will include male and female patients with sickle cell disease (SS,S-beta-Thalassemia) who have previously completed the 16-week treatment period of the double-blind study of bosentan (ASSET 1 or ASSET 2). Patients who meet all the inclusion criteria and none of the exclusion criteria will be started on 62.5 mg bid for 4 weeks and then start the maintenance dose of 125 mg bid (or stay on 62.5 mg if their weight is less than 40kg/90lbs). Patients will be divided into two groups. Group A will consist of patients who begin this study within 4 weeks of completing ASSET 1 or ASSET2. Group B will consist of patients who begin this study longer than 4 weeks after completing ASSET I or ASSET 2. Patients will remain on drug until the FDA approves the drug for use in patients with pulmonary hypertension or until the sponsor decides to stop the study.