A total of 3,254 pregnant women attending two antenatal clinics in Göteborg, Sweden, were screened for bacteriuria. The coverage of the pregnant population in the areas served by the two clinics was estimated to be 88%. Of the women who were registered at the two clinics, 99% took part in at least one screening; 71% were screened during each of the three intervals. The high frequency of screening of the women made it possible to estimate the risk of acquiring bacteriuria during pregnancy. This risk increased with the duration of pregnancy from 0.8% of bacteriuric women in the 12th gestational week to 1.93% at the end of pregnancy. The risk of onset of bacteriuria was highest between the ninth and 17th gestational weeks. The 16th gestational week was the optimal time for a single screening for bacteriuria calculated as the number of bacteriuria-free gestational weeks gained by treatment.