Clinical results from the application of a triple radionuclide subtraction study for preoperative localization of enlarged parathyroid glands are presented in this paper. A scintillation camera image was recorded after i.v. injection of 131I-toluidine blue. The contributions to this image from activity in the thyroid gland and in blood were determined after recording of additional images (obtained after injection of 99mTcO4- and 113mIn, respectively) using an iterative computer algorithm that also provides an estimate for a uniform background. Subtraction of these contributions gives an image that may be interpreted as an image of the selective accumulation of 131I-toluidine blue in the parathyroids. One or more enlarged glands were correctly localized in 17 of 21 cases, but in four cases there were only false positives. In ten cases the correctly localized glands weighed 900 mg or less.