Acute Chlamydia pneumoniae infection may initiate asthma or worsen asthmatic symptoms. In crowded conditions, such as military service, young men are susceptible to respiratory infections, including C. pneumoniae. We recruited 127 asthmatic and 391 nonasthmatic military conscripts, followed up their respiratory tract infections and the kinetics of serum C. pneumoniae antibodies, and assessed the association between C. pneumoniae and asthma during 6 months of military service in 2 intake groups. During the 6-month period, in the July intake group, IgG antibody prevalence decreased from 60.3% to 43.8% in asthmatic and from 55.6% to 22.6% in nonasthmatic conscripts. In the January intake group, IgG antibody prevalence increased from 38.3% to 48.4% in asthmatic and from 37.2% to 43% in nonasthmatic recruits. IgG and IgA antibodies persisted more often in the asthmatic groups. In conclusion, the prevalence of IgG antibodies showed seasonal variation. Military recruits seem to be most vulnerable to C. pneumoniae infections during the period from January to June. The antibody titer changes were more rapid than previously thought.