To determine if there was an association between recto-vaginal group B streptococcus (GBS) colonization and pre-eclampsia, two cross-sectional studies were conducted using statewide hospital databases. The first study analysed data from the state of Florida, USA, and included 190 645 women who were discharged in 2001. This dataset was used to generate the hypothesis that GBS colonization is associated with pre-eclampsia. The second study tested the GBS hypothesis using the records of 577 153 women who delivered in 2004 or 2005 in Texas, USA. Adjusted odds ratios (aOR) for the outcome of pre-eclampsia comparing GBS-positive to GBS-negative women were calculated using logistic regression. The aOR for the association between GBS carriage and pre-eclampsia was 0.71 [95% confidence interval (CI) 0.65-0.77] in the Florida dataset. In the Texas dataset, the overall prevalence of GBS carriage was 14.1% while the overall prevalence of pre-eclampsia was 4.0%. GBS carriers were 31% less likely than non-carriers to have pre-eclampsia (aOR 0.69, 95% CI 0.66-0.72) in Texas. In two large statewide analyses, GBS carriage was inversely associated with pre-eclampsia. A sensitivity analysis revealed that misclassification of GBS status is not a likely explanation of our findings.