In order to elucidate the role of interleukin 8 (IL-8) in the development of chronic lung disease (CLD) of neonates with intra-uterine infection, serial and simultaneous measurements of the concentration of IL-8 and granulocyte elastase alpha 1 proteinase inhibitor complex (E-alpha 1 PI) in the tracheobronchial aspirate of low birth weight infants were conducted. Infants with a high serum IgM level at birth, and who subsequently developed CLD, showed significantly high concentrations of IL-8 and E-alpha 1 PI in the first 48 h. It seemed that IL-8 stimulated neutrophils to release neutrophil enzymes which, in turn, caused the lung tissue injury, resulting in the development of CLD following intra-uterine infection.