Multicenter, placebo-controlled, randomized, double-blind. Subjects are recruited at 12 clinical centers and randomized to 2 mg/kg/day of cyclophosphamide or placebo. Follow-up visits for pulmonary assessments occur every three months for two years after treatment. If patients fail the cyclophosphamide treatment, they will be offered azathioprine for the remainder of the 24 month trial. The primary endpoint of the study is change in forced vital capacity at the end of 12 months of treatment. Secondary endpoints include quality of life, activity, and dyspnea indices, and carbon monoxide diffusing capacity. Recruitment ends in December, 2003.