The authors report 12 cases of chronic pneumonitis of infancy (CPI) studied retrospectively, on autopsy specimens. CPI is a new form of interstitial pneumonia, occurring exclusively is young infants and first described in 1995. Histologically, CPI is characterized by diffuse thickening with hypercellularity of the alveolar septa, associated with hyperplasia of type 2 pneumocytes and accumulation of intra-alveolar macrophages. In all children, initially in good health, the respiratory symptoms initially appeared at the age of 1 to 9 months, with a fatal course despite treatment. Many deaths of young infants, previously attributed to other types of interstitial pneumonia or unknown aetiology, could possibly be due to CPI.