Forty-six patients with cystic fibrosis (CF), 1 month to 32 years of age, had 51 episodes of assisted ventilation. Thirty-five (69%) died after 1 to 395 days of mechanical ventilation and six others (12%) died in the hospital 1 to 42 days after assisted ventilation therapy was discontinued. Only three patients survived longer than one year after discharge. These results are much poorer than those reported for patients with chronic obstructive pulmonary disease of other causes probably because of the different natural history and response to therapy in CF. Patients with CF whose conditions progressively deteriorated despite optimal therapy and no longer respond to antibiotic and physical therapy are not good candidates for mechanical ventilation. The occasional patient with good baseline status in whom acute respiratory failure develops should be considered for assisted ventilation.