When urine, which has been collected by suprapubic bladder aspiration, is appropriately cultured, asymptomatic bacteriuria due to fastidious organisms can be detected quite commonly in apparently healthy pregnant women; Ureaplasma urealyticum and Gardnerella vaginalis can each be isolated from the bladder urine of 10 to 15% of subjects, other bacteria less frequently. Both organisms are often present together, sometimes in addition to "conventional" urinary pathogens. Overall bacteriuria occurs in about 25% of healthy pregnant women, including 6% with "conventional" asymptomatic bacteriuria. Pyuria occurs in about one-third of patients with fastidious bacteriuria. Bacteriuria due to fastidious bacteria occurs significantly more commonly in selected groups of pregnant women. U. urealyticum or G. vaginalis or both can be isolated, generally in relatively high numbers from more than 70% of pregnant women with renal disease and 20 to 30% of those with preeclampsia. In a prospective study in which urine was collected using an open ended catheter, ureaplasmas were isolated 3 times more frequently from women less than 25 years of age than from older women. Women with Ureaplasma bacteriuria at the first antenatal visit were 3 times more likely to develop preeclampsia than those without, but the mean birth weights of the infants born to the two groups of women were not significantly different. Further studies are required to determine the importance of these preliminary findings.