Ultrasonographic prenatal diagnosis of single umbilical artery (UA) is well documented, but the exact siding of the single UA and its correlation with the occurrence of other congenital malformations and the outcome of the baby remain unclear. We report our experience with 46 cases of prenatally diagnosed single UA. This is the first prospective study of a large number of consecutive pregnancies in which the side of the existing artery was identified in fetuses with a single UA. Most of the cases were identified by transvaginal sonography at 14-16 weeks' gestation. A right artery was detected in 25 fetuses (54.3 per cent), and a left artery in 21 cases (45.7 per cent). Six fetuses (13 per cent) had associated anomalies, five of them in the urinary system. No correlation was found between the type or severity of the malformations and the side of the missing (or existing) UA. In our experience, the exact location of the single UA can be reliably determined by ultrasonography from the beginning of the second trimester of pregnancy. The selection process of the missing (or existing) vessel is likely to be random, even though a right single artery was seen slightly more often.