A substantial increase in the prevalence of asthma in the Western world during the last few decades has led to a continuous search for novel factors that might be involved in the development of the disease. We carried out a study to clarify whether there is a relationship between severity of asthma and Chlamydia pneumoniae-specific titres at the group level and whether antibodies to the 60 kDa chlamydial heat shock protein (chsp60) are associated with asthma. A total of 116 (31 men, 85 women) consecutive asthma patients from a chest clinic were recruited and divided into 3 groups according to the severity of the disease: there were 13 asthmatics with severe, 54 with moderate and 49 with mild asthma. In addition, 50 (31 men, 19 women) consecutive blood donors were enrolled to serve as a control group. Sera for the measurements of specific IgG, IgA and IgM antibodies using a microimmunofluorescence test and of chsp60 using an enzyme immunoassay were obtained upon enrolment and also 3-4 months later from the asthma patients. Severe and moderate asthma were found to be strongly associated with elevated IgA antibody levels to C. pneumoniae [odds ratio (OR) 5.58, 95% confidence interval (CI) 1.31-23.72 for severe and OR 5.65, 95% CI 2.05-15.53 for moderate asthma] in a logistic regression model. Furthermore, in women, the occurrence of elevated IgA antibody levels and the age-adjusted geometric mean titres of IgA antibodies were significantly higher among the asthmatics than the controls (p = 0.003 and 0.04, respectively). Antibodies to chsp60 occurred more frequently and in higher concentrations among the asthmatics than the controls, although the differences did not reach significance. In conclusion, severe and moderate asthma were significantly associated with elevated IgA antibody levels to C. pneumoniae suggestive of chronic infection. Antibodies to chsp60 did not prove to be a useful marker of such an infection among the asthmatics studied here.