Thyroid uptake measurements were performed on 246 patients, who underwent radioiodine therapy for benign disease up to 192 hr after oral application of either the test activity (7 MBq) or the therapeutic activity (150-1100 MBq). Using the complete set of uptake values, the cumulated activity in the thyroid was calculated and the dose-to-activity ratio (D/A) deduced. An empiric factor was derived, which allows prediction of the D/A with high precision, using only the late uptake measurement at 96 hr or 192 hr. The correlation between the value calculated from the complete set of uptake values and that of only one at 96 hr or 192 hr was R = 0.99 and 0.97, respectively. The activity required for intended dose can thus reliably be determined by a single, late uptake measurement. In a second analysis, the correlation between the D/A of the test and the therapeutic activity was established. There were two essential findings: For those patients who were without or under stable thyroid-specific medication there was a strong correlation between the two D/A values. The therapeutic value was on the average approximately 15% lower than the test ratio, which is assumed to be due to an enhanced iodine turnover under therapeutic conditions. In patients whose medication changed close to the test study or therapy, the measured test and therapeutic D/A were strongly noncorrelated.