Patients ages 16 and older who have completed the 16-week treatment in the ASSET 1 or ASSET 2 study and who are not pregnant or breastfeeding may be eligible for this study. The research will be conducted in about 25 hospitals in the United States and Europe. Up to 30 participants will be enrolled. The screening visit will involve a physical examination, blood sample of about 3 teaspoons for laboratory tests, and a pregnancy test. Patients' doctors will give them bosentan tablets (62.5 mg each), to take one in the morning and one in the evening. After 1 month, patients will be told whether the dose should be increased to 125 mg tablets to take twice a day. Two weeks after the increase in dose, a blood test will be done to analyze the drug's effects on the liver. After the start of treatment, patients will return for visits every 6 months, when there will be a 6-minute walking test to measure exercise capacity and evaluate shortness of breath. There will be follow-up for patients up to the end of the study and for 28 days after the last dose of bosentan is taken, to collect information about side effects.