We investigated the uptake of therapeutic doses of 131-Iodine in capsular form which were given to 16 patients with benign thyroid disease, and compared it to the uptake of a diagnostic dose of liquid 131-Iodine given to the same patients. The aim of this study was to determine the additional radiation dose sustained by the gastric mucosa, and thus, to establish the safety of this galenic form of 131I. It was found that the average capsule-dissolution time was about 12 min, with a large standard deviation of about 7 min. Using these data and a theoretical radiation-dose calculation, we estimated that the maximum dose to the gastric mucosa was approximately 250 rad (250 cGy) for a therapeutic activity of 5 mCi (185 MBq), which is the maximum dose which may be given as single application to out-patients in Switzerland. Thus, 131I administered in capsular form is a safe galenic form for therapeutic use in patients with thyroid disease.