The objective of the study was to investigate the possible association of Chlamydia pneumoniae (Cpn) in acute exacerbations of chronic obstructive pulmonary disease (COPD) patients. Thirty-eight acutely exacerbated COPD patients and 17 healthy smokers were enrolled in the study, as the study and control groups respectively. Nasopharyngeal swabs and paired serum samples for antibody testing of Cpn (microimmunofluorescence--MIF) were obtained from all subjects. Sputum cultures of COPD patients were also performed. No pathogenic bacteria were isolated from nasopharyngeal swabs in any subject. Serologic evidence of recent Cpn infection was observed in 13 (34%) COPD patients and in one (5%) control subject. The prevalence of Cpn IgG and IgM antibodies representing acute infection were significantly higher in COPD patients than in control subjects (P < 0.05 and P < 0.01 respectively). Prevalence of IgA antibodies and IgG pre-existing antibodies did not show any difference (P > 0.05). Microbiologic culture of the sputa yielded potentially pathogenic micro-organisms in 23 of 38 (60%) COPD patients. Alpha-haemolytic streptococcus (35%), Niesseria spp. (31%) and Candida spp. (9.5%) were most prominent micro-organisms in positive cultures. Although a high prevalence of IgG antibodies against Cpn was detected, it was the sole causative agent in only four (10%) patients. We conclude that a remarkable number of COPD patients (34%) are acutely infected with Cpn and it may either be the sole causative agent or frequently a co-agent in acute exacerbations.