scintigraphic method for the estimation of the thyroid mass in patients with Graves' disease is described. The method was first standardized using thyroid phantoms with eight different volumes ranging from 5 to 110 cm(3). The planar and single photon emission computed tomography (SPECT) images of each phantom were acquired with four different activities [3.7 MBq (100 microCi), 11.1 MBq (300 microCi), 22.2 MBq (600 microCi) and 37 MBq (1.0 mCi) of 99mTc-pertechnetate] with a 20% window symmetrically placed over the photopeak of 99mTc. The thyroid lobes were enclosed with the help of regions of interest (ROI) tools and a threshold was selected to identify the thyroid boundaries. The same threshold was used in all slices of an image. In the phantom study, a 20% threshold for planar images and a 30% threshold for SPECT were found to be optimum for measuring the thyroid volume. The volume from planar images was calculated by the formula described by Allen and Goodwin (The scintillation counter as an instrument for in-vivo determination of thyroid weight. Radiology 1952; 58: 68-79), whereas, in SPECT images, the sum of the slice areas was multiplied by the slice thickness. The estimated volume of each phantom was compared and correlated with its actual volume. After standardization of the technique with phantom studies, planar scintigraphy (with 20% threshold) in 51 patients and SPECT (with 30% and 35% threshold) in 40 patients with Graves' disease were performed to estimate the thyroid size. The thyroid size was also estimated by ultrasonography, which showed good agreement with the scintigraphic method, particularly with SPECT.