In this matched-pair study 139 pregnant women were matched on the basis of age to an equal number of non-pregnant women with no signs of genital infection. The mean age was 28.7 years (range 20-41). The cut-offs used for detection of chlamydial antibody were 1:64 and 1:128 for IgG and 1:16 for IgA. IgG antibody at 64 was detected in 37.4% of pregnant women, compared to 46% of controls (p = 0.145). There was, however, a statistically significant difference between the groups for IgG at 128 (gravidae = 15.8%; controls = 28%; p = 0.014). IgA were detected in 8.6% and 16.5% of subjects, respectively (p = 0.047). IgG levels did not vary with increasing age among the pregnant women, but rose significantly with age in non-pregnant controls (logistic regression p-values = 0.011 and 0.006, for IgG at 64 and 128, respectively). IgG-positive women in the control group tended to be older than pregnant IgG-positive women (p = 0.06). These differences could not be explained by marital status, parity or use of oral contraceptives. In view of the lack of epidemiological differences, biological explanations might be invoked.