44 out of 50 patients with idiopathic fibrosing alveolitis were followed up from 1 month to 11 years (mean 2 years 9 months). One patient out of 13 (8%) with 4 symptoms or fewer (mean follow-up 55 months) died. 23 out of 31 (74%) with 5 symptoms or more (mean follow-up 24.3 months) died, and 68% of fatalities occurred within 2 years. Lung function tests were performed in 47 cases and histological diagnosis in 38. Correlations between symptoms, lung function, and roentgenological and histological patterns are discussed. Two circumstances dramatically increased lethality: occurrence of 5 symptoms or more in a period of less than 2 years.