Aim: To investigate long-term effects on children previously infected with Ch lamydia pneumoniae. Methods: A followup questionnaire was sent to all participan ts from a former population-based study in order to investigate health status d uring the 4 y that had elapsed between the two studies. In the original study, t he prevalence of C. pneumoniae infection was 23%as determined by PCR analyses o n throat swab specimens. These PCR results were found to have no detectable corr elation for clinical disease. The main outcome measures in this follow-up study were the reported prevalence of respiratory tract infections, asthma and allerg y. Results: Approximately 83%completed the follow-up questionnaire. No increas e in respiratory tract infections was reported by children previously found to h ave C. pneumoniae infection. A diagnosis of allergy was more common in the forme r PCR-negative population (13.4%vs 4.7%, p<0.03). The differences were most a pparent in the population with atopic heredity. In a logistic regression model w ith different suggested risk factors for allergy, earlier infection with C. pneu moniae reduced the risk for allergy (OR = 0.13; 95%CI: 0.02-0.99). This was no t found for asthma. Conclusion: A positive PCR test for C. pneumoniae in young c hildren was associated with a lower risk of developing allergic airway disease i n this study population, and did not predict an increase in respiratory tract in fections. A: To investigate long-term effects on children previously infected with Ch lamydia pneumoniae. Methods: A follow up questionnaire was sent to all participant ts from a former population-based study in order to investigate health status d uring the 4 y that had elapsed between the two studies. In the original study, t he prevalence of C. pneumoniae infection was 23% as determined by PCR analyzes on throat swab specimens. These main outcome measures in this follow-up study were the reported prevalence of respiratory tract infections, asthma and allerg y. Results: Approximately 83% completed the follow-up questionnaire. No increas e in respiratory tract infections was reported by children previously found to h ave C. pneumoniae infection. A diagnosis of allergy was more common in the for PCR PCR-negative population (13.4% vs 4.7%, p <0.03). The differences were most a pparent in the population with atopic he redity. In a logistic regression model w ith different suggested risk factors for allergy, earlier infection with C. pneu moniae reduced the risk for allergy (OR = 0.13; 95% CI: 0.02-0.99). This was no t found for asthma. Conclusion: A positive PCR test for C. pneumoniae in young c hildren was associated with a lower risk of developing allergic airway disease in this study population, and did not predict an increase in respiratory tract in fections.