Mucoid impaction of the bronchi (MIB) is an unusual complication of adult asthma. In a review of 675 serial chest radiographs of 68 children and 2 adults with cystic fibrosis (CF), opacifications consistent with the diagnosis of MIB have been noted in 22 children and 1 adult, an overall incidence of 33%. The lesion of MIB in CF is a bronchusrelated complex of branched finger-like or nodular densities due to progressive distention of bronchi with tenacious mucopus. It is most commonly observed in the right upper lobe. Although it may spontaneously disappear, it tends to recur. Persistence of MIB for periods longer than 1 month can be related to subsequent bronchiectasis in the same segment. Clinical diagnosis is totally dependent on radiographic recognition, since symptoms and signs are characteristically lacking or non-specific. Atelectasis does not occur, probably because of collateral ventilation from adjacent uninvolved lung. Certain apparent differences have been noted between the adult (asthma) and childhood (CF) forms of MIB. Planigraphic and bronchographic studies in selected patients confirm the intrabronchial location of this process. Bronchial lavage is effective in washing out some impactions that have not responded to more conventional mucolytic and physical therapy. Recognition of MIB in a roentgenogram of a child should be considered presumptive evidence of CF, since similar opacifications are very rarely seen in children with other diseases. We believe that (1) progressive MIB at a subsegmental level is the major cause of bronchiectasis in CF and (2) secondary collateral ventilation may be the major cause of obstructive overinflation of the lungs in CF.