The aim of the present study was to determine ultrasonographically the size of the umbilical structures in clinically health newborn calves and to evaluate their involution during the first three weeks of life. A total of 13 ultrasonograms were obtained in each of 20 healthy newborn calves using a 7.5-Mhz sector scanner transducer placed across the linea alba of the abdominal wall. The umbilical vein was imaged at five defined cross sections from the umbilicus to the portal vein in the liver, whereas the urachus and the umbilical arteries were imaged at four defined cross sections from the umbilicus to the pelvis. The umbilicus was scanned at three defined locations. All ultrasonographic cross sections were compared to those of a frozen newborn calf. The results were further tested against in a single examination of 20 different calves between 2 hours and 34 days. Standardized ultrasonographic examination is a safe and reliable method to image the umbilical structures in calves. The umbilical vein was imaged between the umbilicus and the caudoventral margin of the liver in calves up to three weeks of age. The umbilical vein, which at first was oval and thin-walled with a wide anechoic center (vertical diameter 11 +/- 3 mm and horizontal diameter 19 +/- 7 mm), diminished in diameter during the first week of life to 5 to 8 mm. The course of the umbilical vein in the liver was imaged in calves up to two weeks of age. The umbilical arteries could only be clearly imaged during the first seven days of life. They had a mean diameter of 8 +/- 2 mm and were seen as round, thick walled vessels with an echogenic to hypoechoic center. In all calves, the urachus was not well-defined and was difficult to image. The data of the umbilical structures accumulated in this study can be used as a reference in the diagnosis of abnormalities of the umbilical structures in young calves.