The syndrome of infantile bronchiolitis in cystic fibrosis (CF) carries a high mortality. Fifteen cases of CF encountered over the past 19 years with severe bronchiolitis with onset during the first 6 months of life are described. Treatment include steroids in high doses. All patients recovered. Further progress resembled the usual natural course of CF and showed no evidence of persisting lung damage. The mechanism of this syndrome is not clear and is probably dependent on many factors involved in early lung disease in CF. The frequency of severe bronchiolitis in cystic fibrosis may not be high, but it continues to be seen in clinical practice today.