Associations have been reported between Chlamydia pneumoniae seropositivity and both acute and chronic obstructive airway diseases. Plasma specimens collected between 1979 and 1983 from 1, 773 men 45 to 59 yr of age in Caerphilly, South Wales, were tested for IgG and IgA antibodies to C. pneumoniae (TW183) by microimmunofluorescence. Subsequent mortality and medication for obstructive airway disease were ascertained at 5-yr follow-up examinations. Spirometry was performed at the first and second examinations and analyzed both cross-sectionally and longitudinally; 642 men (36%) had IgG antibodies at a titer of 1:16 or above, of whom 362 also had detectable IgA antibodies. No statistically significant associations were found between either IgG titer or IgA titer and any of the outcome measures: inhaler therapy at entry; commencement of inhalers during follow-up; death from respiratory causes; baseline FEV(1), FVC, and FEV(1)/FVC ratio; and decline in FEV(1) (p > 0.1 throughout). Men with high IgG titers (>/= 1:64) had a slower rate of decline of FEV(1) than did seronegative subjects (adjusted mean difference in 5-yr change in FEV(1): +22 ml, 95% confidence interval: -31 ml to +76 ml). Men with high IgA titers (>/= 1:16) had a slightly faster rate of decline (-12 ml, - 96 ml to +71 ml). This first prospective assessment suggests that chronic C. pneumoniae infection is not a major risk factor for progressive airflow obstruction.