The activity of 131I to be administered as therapy to patients with thyroid disease is usually calculated from 24 h radioiodine uptake and thyroid volume. The aim of the present study was to compare thyroid volume, measured by scintigraphy and ultrasonography, to evaluate the impact of these methods on the calculated 131I dose. Forty patients (20 with diffuse toxic goiter and 20 with multinodular toxic or nontoxic goiter) were investigated. On the same day, thyroid volume was measured by ultrasonography (using transverse scans at 5 mm intervals) and by scintigraphy, using either the ellipsoid formula (SC-E: [symbol: see text]/6 x height x width x depth) or the Himanka formula (SC-H: 0.33 x (planimetric surface in pixels)3/2). With ultrasonography, the size of diffuse goiters was smaller than that of nodular goiters (median values and range: 18 ml (11-46) and 50 ml (14-198) respectively, P < 0.001). Both scintigraphic methods, however, failed to demonstrate a significant difference between diffuse and nodular goiter size. In patients with diffuse goiter, thyroid volume measured by SC-E did not differ from that measured by ultrasonography, whereas thyroid size was overestimated by 53% using the Himanka formula. In contrast, in patients with nodular goiter, thyroid volume measured by SC-H did not differ from that measured by ultrasonography, whereas the ellipsoid formula underestimated thyroid size by 48%. The overestimation of diffuse goiter size by the Himanka formula resulted in a relatively modest median excess of 96 MBq (range -118 to +248 MBq) of the calculated 131I dose. The underestimation of nodular goiter size by the ellipsoid formula resulted in a calculated dose that was 278 MBq lower (range -1624 to +141 MBq). The median calculated 131I dose based on the Himanka formula was not different from that based on ultrasound, but large differences in calculated 131I dose (up to 1280 MBq) were found in individual cases. In conclusion, thyroid volume can be assessed with accuracy by scintigraphy using the ellipsoid formula in patients with diffuse goiter. Wide differences, however, are observed in the size of nodular goiters measured by scintigraphy and ultrasonography.