Pneumonia that occurs within 28 days of the onset of measles rash is a common cause of severe pulmonary morbidity and/or death among poor children. The prevalence of such pneumonia can be related to the effectiveness of measles immunization programs. For 20 of 57 new cases of bronchiectasis in children undergoing bronchography, a strong causal relationship to measles was found. The lungs of 21 unselected children who died in the wake of measles were examined. Severe necrosis of bronchi and bronchioles was found in those children who had developed intercurrent adenovirus and herpesvirus infections. Bacterial suppuration produced a less severe necrosis. It is suggested that intercurrent adenovirus and herpesvirus infections that occur following measles are the most important initiating causes of follicular bronchiectasis in childhood. The severity of these supervening infections may be mediated by the transient immune suppression that occurs as a consequence of both measles and inadequate nutrition.