After receiving between 100 and 300 mu Ci of the positron-emitting radioisotope 124I (half-life, 4.2 days), 64 patients with a variety of thyroid disorders were imaged with a high resolution positron camera. A 3-dimensional image of the distribution of radioiodine uptake within the thyroid was obtained from a single 10- to 15-min scan. This image may be viewed as a sequence of 2-mm thick transverse, sagittal, or frontal sections or as a 3-dimensional shaded surface. The functional volume of the thyroid may be estimated by counting the volume elements (voxels) inside the thyroid surface. The precision of the estimate varied from 6-15%, depending on the size and clinical status of the thyroid. The volume estimation procedure was validated with phantoms and with the thyroids of patients who subsequently underwent partial thyroidectomy. This 3-dimensional imaging technique may be useful for diagnosis and management of thyroid diseases.