We hypothesized that early uptake of iodine-131 (131I) at 3-6 hr (EU) by the thyroid could be used to calculate 131I therapy with results comparable to those obtained using late uptake of 131I at 20-28 hr (LU) results. A retrospective study was undertaken. Twenty-seven patients with untreated thyrotoxic Graves' disease were given 3-5 microCi of 131I and EU and LU were determined with an uptake probe. We derived a "best fit" curve on plots of EU (X-axis) and LU (Y-axis). The equation for the curve (LU = -55.7 + 73.2 log EU) was used to predict late uptake (PU) from EU results on a second group of 24 similarly defined Graves' patients. PU and measured LU were then applied to 131I treatment calculations in these 24 patients. PU correlated closely with LU (r = 0.94). Dose calculations based on PU and LU gave very similar results (r = 0.97). Using this method, same day diagnosis and treatment of Graves' is achievable.