To evaluate the long-term effect of nocturnal mist tent therapy on the progression of airway obstruction in children with cystic fibrosis (CF) of varying severity, two matched groups each consisting of 24 children with CF were studied during 18 months on mist tent therapy and 18 months of therapy. The progression in airway obstruction was measured by change in serial measurements of maximal midexpiratory flow (MMEF), from which a regression equation of MMEF against time was obtained for each individual. Changes in MMEF value with or without therapy were compared in patients matched for severity of disease as indicated by initial MMEF values. For the group as a whole no differences were found in the progression of the airway obstruction whether the patients received mist tent therapy or not. This therapy failed to benefit any of the groups of children with CF who had early, moderate, or advanced airway obstruction as judged from their initial MMEF value. It is concluded that nocturnal mist tent therapy neither decreases airway obstruction nor prevents its progression in children with CF.