Infection with Chlamydia pneumoniae may be associated with an increased risk of lung cancer. We conducted a matched case-control study (508 pairs) nested within a large prospective study to investigate whether IgA antibody titers to C. pneumoniae measured by the microimmunofluorescence test are associated with lung cancer risk after controlling for confounders. Individuals with antibody titers > or = 16 had 1 VSports手机版. 2 times the risk of lung cancer (95% confidence interval, 0. 9-1. 6) compared to those with lower titers. There was a significant trend (P = 0. 007) of increasing odds ratios with increasing IgA titers primarily due to an odds ratio of 2. 8 (95% confidence interval, 1. 1-6. 7) associated with titers > or = 256. Lung cancer risk associated with IgA titers > or = 16 was stronger among former smokers. To better understand predictors of IgA seropositivity, we also examined demographic, lifestyle, dietary, and medical correlates of IgA titers > or = 16 among controls. Those with race not classified as White or Black were more likely to have IgA titers > or = 16; there were no significant differences in seropositivity by smoking behaviors. In summary, the adjusted odds ratio for lung cancer associated with IgA titers > or = 16 was compatible with a weakly positive association, although nondifferential measurement error of antibody titers may have resulted in a conservative bias. Future studies using precise measures of chronic C. pneumoniae status are needed to better determine the role of this organism in the etiology of lung cancer. .