OBJECTIVE To identify risk factors for group B streptococcus (GBS) colonization in pregnancy, hypothesizing that health care workers may have increased risk. METHODS Population-based, case-control study comparing 40,459 cases of GBS colonization, identified from Washington State birth certificate data linked to hospital discharge data for live births between 1997 and 2002, with 84,268 controls matched by year of delivery by multivariable logistic regression. RESULTS After adjustment for confounders, the following characteristics were independently associated with increased maternal GBS colonization: health care occupation (odds ratio [OR] 1.22, 95% confidence interval [CI] 1.07-1.38), black race (OR 1.54, 95% CI 1.36-1.74), overweight (OR 1.07, 95% CI 1.01-1.12), obesity (OR 1.20, 95% CI 1.13-1.28), severe obesity (OR 1.45, 95% CI 1.28-1.63), median income greater than lowest quintile (OR 1.29, 95% CI 1.20-1.39 for fifth versus first quintile), some high school education (OR 1.21, 95% CI 1.05-1.40), high school graduate (OR 1.35, 95% CI 1.16-1.56), and adequate prenatal care (OR 1.14, 95% CI 1.06-1.24). Hispanic women (OR 0.88, 95% CI 0.80-0.96) and smokers (OR for 1-10 cigarettes per day 0.90, 95% CI 0.83-0.97) had a decreased odds of colonization. CONCLUSIONS Health care workers, black women, and women with high body mass index may be at greater risk of GBS colonization in pregnancy. However, any increases in risk are modest and the association between a health care occupation and GBS colonization needs to be investigated further.