Asymptomatic bacteriuria is thought to be associated with complications during late pregnancy or the puerperium. To understand this association, the underlying condition must be denned. This report describes findings in 53 women with asymptomatic bacteriuria early in pregnancy. Using a bladder washout tcchnic, the origin of bacteria was localized to the kidneys in 13 women (24.5%) and to the bladder in 19 (35.8%); in 7 (13.2%), the source was indeterminate. In the remaining 14 (26.4%), urine was spontaneously sterilized in an average period of 9.3 days after the screening test. Bacteriuria during pregnancy is obviously a heterogeneous entity, and it seems reasonable to anticipate that complications arise in only a particular subgroup of this population. This subgroup needs to be identified for rational prophylaxis and followup.