A cohort of 200 women with singleton pregnancies attending the antenatal clinic, were studied to determine the prevalence of genital mycoplasma and ureaplasma infections in pregnant women and the effect of infection on pregnancy outcome. Vaginal and endocervical swabs were taken at 26-30 wk and again at 36-38 wk of pregnancy and cultured for mycoplasma and ureaplasma. Forty (20%, 95% CI 14.5-25.6%) women were found to be infected with arginine metabolising mycoplasmas and 79 (39.5%, 95% CI 32.7-46.3%) with ureaplasma at either site or either time. The mean +/- SD birth weight of infants born to women infected with mycoplasma alone was 2879 +/- 471 g while that of infants born to women infected with ureaplasma alone was 2964 +/- 412 g. Mean +/- SD birthweight of infants born to women infected with both organisms was 2969 +/- 389 g while that of infants born to uninfected women was 2919 +/- 432 g. These differences were not statistically significant. The median gestation at delivery was 39 wk in the cohort; the differences among the groups were minor and statistically not significant. Thus, although genital mycoplasma/ureaplasma infections were frequent, no association was observed between infection and pregnancy outcome.