Simple clinical measurements were performed on a group of 23 very low birth weight babies (less than 1500 g and less than 32 weeks gestation), and combined with pulmonary function studies in 13 cases. The studied included a special single-breath analysis, or Tidal Resistance Profile, which enabled us to detect air trapping. 2 infants developed classical Wilson-Mikity syndrome and 2 further infants exhibited a milder, shorter form of the same disorder. All 4 survived. Results suggest that all infants in this maturity group are at risk of developing Wilson-Mikity syndrome, and that it comprises a wide spectrum of respiratory dysfunction.