Abstract Chlamydiae are obligatory intracellular gram-negative bacteria with a unique growth cycle. They are very successful pathogens and responsible for a wide variety infections in humans and different animal species. In addition, they have a tendency to cause recurrent, persistent or chronic infections with potentially severe sequelae years or decades later. The general purpose of this work was to study the possible serological associations between chronic chlamydial infection, systemic inflammation and reproductive health in a general population. The chlamydial heat shock proteins 60 and 10 (Hs10 and Hsp60) have been suggested to contribute to the pathogenesis of chronic chlamydial infections. Thus, the antibodies to chlamydial Hsp10 and Hsp60 were also investigated in complications of pregnancy. The present study was a longitudinal population-based birth cohort study, and all of the original papers of this dissertation are based on a nested case - control design. Our results confirmed the serological association between C. trachomatis infections and subfertility and the rather high incidence of undiagnosed C. trachomatis infections in the male partners of subfertile couples. We further demonstrated a serological association between previous C. trachomatis infections, immunity to chlamydial Hsps and female subfertility. We also showed that serological markers of chronic chlamydial infection present as early as the first trimester are associated with preterm delivery among nulliparous women. When elevated levels of C. trachomatis IgG and hsCRP were present, the estimated risk for preterm delivery was over 4-fold. According to our study, nulliparous women who subsequently developed preeclampsia leading to preterm delivery, which was used as a marker of more serious illness, had significantly more often serum IgG antibodies to C. pneumoniae during the first trimester of pregnancy compared to the preeclamptic women who delivered at term. In conclusion, chronic C. trachomatis infection was found to associate with subfertility both in men and in women. In addition, a subclinical chronic inflammatory process associated at least partly with chronic C. trachomatis infection and present in the first trimester already may be important in the development of preterm delivery. Chronic C. pneumoniae infection and systemic low-grade inflammation were found to associate with pregnancies that lead to preeclampsia and preterm delivery.