Two radiologists individually studied 220 intravenous urograms done during pregnancy in patients with apparently normal urinary tracts, and an observer-agreement technique was used to obtain the results. The few cases seen in early pregnancy show that dilatation of the urinary tract is uncommon, or at least mild, before mid term. Shortly thereafter, however, dilatation appears abruptly and remains virtually unchanged both in incidence and degree until full term. After mid term, the right side is dilated in about three-quarters of cases and the left side in about one-third. In only 10 % was the left side fuller than the right; in 86%, the right side was the fuller. Severe dilatation is infrequent, especially on the left. The dilatation never extends into the pelvis. The abdominal ureters are displaced outwards in a minority of cases, more often on the left. Little or no association was found between the dilatation and fetal position, maternal gravidity or urinary infection. These statistically derived conclusions should provide a firmer basis for what constitutes the limits of normality in the intravenous urogram during pregnancy.