In a nested case-control study, the occurrence of Ureaplasma urealyticum in cervical specimens from 84 women with idiopathic preterm delivery and from 400 women delivering at term was investigated. The two potential risk factors for preterm delivery, colonization with Ureaplasma urealyticum and bacterial vaginosis, were found to be interdependent variables. The association between these factors and preterm delivery was assessed by regression analysis. Neither colonization with Ureaplasma urealyticum (odds ratio [OR] 0.7, 95% confidence interval [CI] 0.4-1.2) nor bacterial vaginosis (OR 0.8, 95% CI 0.3-1.8) was associated with preterm delivery. In women who delivered preterm, biovar 2 was found significantly more often in those with the clinical diagnosis of bacterial vaginosis (43%) than in those without (5%) (OR 15, 95% CI 1.2-209).