To elucidate the mechanism of the development of chronic lung disease (CLD) in infants without respiratory distress syndrome or intra-uterine infection, we serially measured the concentrations of interleukin 8 (IL-8) and granulocyte elastase alpha 1 proteinase inhibitor complex (E-alpha 1 PI) and elastase activity in the tracheobronchial aspirate of very low birth weight infants without respiratory distress syndrome or intra-uterine infection until day 28. IL-8 concentration and elastase activity between day 21 and 28 in infants who developed CLD later were significantly higher compared with those in infants who did not develop CLD. E-alpha 1 PI concentration between day 25 and 28 in infants who developed CLD later was significantly higher compared with those in infants who did not develop CLD. The area under the curve of the IL-8 and E-alpha 1 PI concentrations and elastase activity between day 1 and day 28 in infants with CLD was significantly higher than those in infants without CLD. These data suggest that the lung tissue injury caused by the enzymes from neutrophils accumulated and activated by IL-8 also play an important role in the development of this type of CLD.