Serum immunoglobulin M (IgM) antibodies to Chlamydia trachomatis and to human cytomegalovirus (CMV) were detected by enzyme-linked fluorescence assay and enzyme-linked immunosorbent assay, respectively in 19 premature infants with chronic lung diseases, in 43 extremely low birth weight premature infants and in 123 neonates with elevated serum IgM levels. Ten of the 19 premature infants with chronic lung diseases had elevated serum IgM levels, and five had IgM antibodies to Chlamydia trachomatis. Three of the 43 extremely low birth weight premature infants had elevated serum IgM levels, and two had IgM antibodies to Chlamydia trachomatis. Three of the 123 neonates with elevated serum IgM levels (excluding those with chronic lung diseases and extremely low birth weight) had IgM antibodies to CMV. These results suggest that chronic lung diseases in low birth weight infants might be caused by intrauterine Chlamydia trachomatis infection.