At follow-up, patients given sildenafil could walk longer distances in 6 minutes and had greater decreases in pulmonary arterial pressures than did those given the dummy pills. Neither sildenafil nor the dummy pills affected breathlessness symptoms. None of the 134 patients given sildenafil died; however, 7 of the 131 patients given dummy pills died. More patients given dummy pills required changes in epoprostenol doses because of clinical deterioration. More patients given sildenafil reported side effects, such as headaches and abdominal discomfort (dyspepsia).