To determine whether recently diagnosed adult-onset asthma is associated with serologic evidence of chronic Chlamydia pneumoniae infection, we performed a case-control study in a primary care clinic of cases with asthma (25 adults reporting first symptoms of asthma within 2 years of enrollment) and 45 concurrently enrolled sex and age (+/- 10 years) matched non-asthmatic controls with normal pulmonary function. C. pneumoniae-specific IgA, IgG and IgG4 antibodies, and circulating immune complexes (CIC) were measured by microimmunofluorescence testing. Results showed that frequencies of IgG titres > or = 16 (92%), IgG4 titres > or = 16 (20%) and CIC > or = 4 (60%) in asthma patients were not significantly different from those of controls. However, asthmatics had a significantly higher prevalence of C. pneumoniae-specific IgA titres > or = 10 (72% of cases vs 44% of controls, P < 0.05). After adjustment for the effects of age, sex and smoking, the odds ratio for an association of IgA and asthma was 3.7 (95% confidence interval 1.2-11.5). We conclude that recently symptomatic reversible airway obstruction in adults is associated with the presence of C. pneumoniae-specific IgA antibodies, a proposed indicator of chronic respiratory C. pneumoniae infection.