Mycoplasma pneumoniae and Chlamydia pneumonie are important etiological agents responsible for human respiratory tract diseases. Recently, these atypical microorganisms received much attention regarding their role in bronchial asthma pathogenesis, which is one of the most frequent chronic diseases in children. The aim of the study was to investigate the association between infections caused by these pathogens and respiratory tract diseases in children. Levels of M. pneumoniae and C. pneumoniae-specific antibodies were determined in serum samples obtained from 30 patients suffering from bronchial asthma exacerbations, 10 patients with pneumonia, 28 patients with chronic otitis media with effusion (COME) and 22 sinusitis patients. Specific anti-M. pneumoniae antibodies were detected more frequently in the patients enrolled in the study than in control subjects. The highest percentage of the serum samples, which demonstrated the presence of M. pneumoniae-specific antibodies was demonstrated in patients with asthma (60%) and it was twofold higher than in control subjects. Serologic profile of 26.6% patients with asthma, 50% of patients with pneumonia, 39.2% of patients with COME, 45.4% of patients with sinusitis and 10% of control subjects was consistent with a possible acute infection caused by M. pneumoniae. The presence of specific anti-C. pneumoniae antibodies was demonstrated in a smaller percentage of patients--in 13.3% of children with asthma, 10% of children with pneumonia and in 7.1% of patients with COME; the level of specific antiobodies was suggestive of acute chlamydial infection only in COME patients. Analysis of serologic markers for atypical bacteria infections indicates a possible association between infections caused by M. pneumoniae and bronchial asthma exacerbations and other respiratory tract disorders including pneumonia, sinusitis and COME.