Data on 622 patients with cystic fibrosis born in Victoria, Australia from 1955 to 1980 and on 344 surviving patients in the care of a specialist clinic on June 30, 1983 were analyzed for factors associated with better survival and a less rapid progress of lung disease. Presentation with predominantly gastrointestinal symptoms, other than meconium ileus, was associated with an improved prognosis, whereas presentation with predominantly respiratory symptoms was associated with a worse prognosis. While infants diagnosed before the age of 6 months as a result of routine testing because of a family history of the disease seemed to have less rapid progress of lung disease, their ultimate survival did not seem to be better than that of patients presenting symptomatically after the newborn period. The sex of the patients did not appear to have prognostic significance. Age at diagnosis did not affect rate of progress of lung disease or survival when infants dying within 6 months of birth were excluded. There was a close association between the extent of lung disease at diagnosis and current lung disease. Failure to reverse extensive disease at diagnosis or deterioration of lung disease in the first year after diagnosis was associated with a less favourable course.