The purpose of this study was to determine the usefulness of serum KL-6 (sKL-6) level as a clinical marker, in terms of pulmonary function, for infantile chronic lung disease (CLD). The study population comprised 23 infants less than 31 weeks of gestational age admitted to the neonatal intensive care unit of Kobe University Hospital between March 2002 and August 2003. Blood samples were obtained on day 1, 3, 7, and then weekly until 36 weeks. sKL-6 was measured by means of electro-chemiluminescence immunoassay. For evaluation of pulmonary function, static respiratory system compliance (Crs) was measured on the same day. There were nine infants with CLD and 14 without (non-CLD). Peak sKL-6 levels of the non-CLD infants were less than 200 U/ml except for two cases, while those of CLD infants were over 200 U/ml except for one case. Infants of earlier gestational age, exposed to longer mechanical ventilation and with reduced pulmonary function had the higher sKL-6 levels. However, there was no significant relationship between sKL-6 level and Crs at any postnatal age.