This study evaluated the relationship of vaginal Escherichia coli colonization to birth weight < 1500g and other perinatal complications in a cross-sectional study of 2646 women at the University of Washington Medical Center, Seattle, between October 1992 and January 1995. Vaginal E. coli colonization was more strongly associated with delivery at <34 weeks (relative risk [RR], 1.7;95% confidence interval [CI], 1.3–2.3) and very low birth weight (RR, 1.9; 95% CI, 1.3–2.7) than with prematurity between 34 and 36 weeks or low birth weight. Heavy growth of E. coli had a higher risk of very low birth weight than light growth (RR, 2.4; 95% CI, 1.0–6.2). It may be important to screen and treat pregnant women for genital tract colonization with E. coli during prenatal care.