--Disease Characteristics-- Cystic fibrosis diagnosed as follows: Sweat sodium or chloride greater than 60 mEq/L by pilocarpine iontophoresis or cystic fibrosis genotype Clinical manifestations Estimated 2-year survival greater than 50%, i.e.: FEV1 at least 30% of predicted PaO2 greater than 55 mm Hg on room air PaCO2 less than 50 mm Hg on room air --Prior/Concurrent Therapy-- At least 2 months since systemic glucocorticoids At least 90 days since participation in investigational therapeutic study --Patient Characteristics-- Pulmonary: No pneumothorax within 12 months No asthma or allergic bronchopulmonary aspergillosis requiring glucocorticoids within 2 months No sputum pathogens unless sensitive to at least 2 antibiotics No hemoptysis of more than 250 mL blood over 24 hours within 1 year Other: No active adenoviral infection Ad5 (or similar type) antibody seropositive No other contraindication to protocol participation, e.g.: Drug abuse Alcoholism Psychiatric instability Inadequate motivation Documented azoospermia (men) Bilateral tubal ligation or hysterectomy (women) Screening exams within 4 weeks prior to registration