Infection is presumed to have a rule in the promotion of asthma exacerbations, and in deterioration of the course of the disease, Chlamydia pneumoniae (C. pneumoniae) is claimed to be a possible cause for these two issues. To assess the positivity and the titer of C. pneumoniae IgG antibodies in relation to the state of asthma and its severity. 61 asthmatic patients aged 15-85 years (mean of 47.10±14.887), and 29 apparently healthy, nonasthmatic age and gender-matched volunteers (control group) were assessed as at Asthma and Allergy Clinic in Al-Sader Medical City in AL-Najaf province, Chlamydia Pneumoniae IgG ELISA Kit was used for the detection of IgG antibody to C. Pneumoniae in human serum to detect chronic infection, and Spirometric test was done, and the best results for FEV1 and PEFR were taken. IgG antichlamydial antibodies were positive in 21 (34.4%) of patients compared to 4 (13.8%) of controls, and the difference was significant with OR=3.281, the seropositivity in acute exacerbation was more than in stable asthmatic, 43.8% vs. 24.1% (P=0.029), seropositivity was nonsignificantly more in moderate and severe asthma as compared with mild asthma, a significant inverse correlation between IgG titer and pulmonary function test parameters (FEV1, PEFR) was observed as the FEV1 & PEFR values decrease with increase IgG titer. Chronic C. pneumoniae infection is common in adult asthmatics and correlated with exacerbations & increased severity and disturbed lung function.