The effect that vaginal lactobacilli have on trichomoniasis is not known. At 3 clinics for treatment of sexually transmitted disease, we recruited 521 female patients with trichomoniasis and 176 control subjects. All participants underwent physical examinations and testing for sexually transmitted infection and completed questionnaires. Low levels of vaginal lactobacilli were associated with trichomoniasis (odds ratio [OR], 2.2 [95% confidence interval {CI}, 1.4-3.2]). After adjustment for covariables, this association remained in women with a higher educational level (OR, 4.6 [95% CI, 2.2-9.5]) but not in women with a lower educational level (OR, 1.6 [95% CI, 0.7-3.4]). Vaginal lactobacilli may be associated with trichomoniasis in women with higher levels of education or a related behavioral factor.