Using ultrasound it is possible to determine the thyroid volume with a maximal error of 12%. The method is based on the electronic planimetry of a sequence of transverse ultrasound scans of the thyroid. Following this, the "slices" are added using a small computer. On 101 patients five commonly used methods of volume determination were compared with this reference technique. The volume estimation by palpation and interpretation of the scintigram as well as the volume calculation from the scintigram area or from an ellipsoid model gave maximal errors between plus or minus 82% and plus or minus 90%. The combination of scintigram area or scintigraphically determined length and width of a single thyroid lobe and the sonographically measured thickness reduced the maximal error to plus or minus 35% and plus or minus 37%. In 10 patients teh reproducibility of the reference technique was excellent and the absolute maximal error was 4.6%, even when the patients were differently positioned.