A retrospective study was performed to assess the feasibility of using the 4-hour I-123 early uptake value in the calculation of the treatment dose of radioactive iodine in Graves' disease patients. The Graves' disease patients were randomly divided into two groups. Data from the first group of 35 patients were used to obtain a regression relationship between the early 4-hour uptake (EUp) and 24-hour late uptake values (LUp). This equation, LUp = -38.618 + 65.216 Log (EUp) was used to estimate the predicted late uptake (PUp) from (EUp) in the second group of 34 patients. LUp was used to calculate the I-131 therapy dose for treatment in these patients. In 28 clinically euthyroid patients, the 4-hour uptake I-123 radioactive iodine uptake (RAIU) ranged from 7% to 20% (mean, 12 +/- 3.5%). In 60 Graves' disease patients, the 4-hour RAIU ranged from 15% to 100% (mean, 57 +/- 24.8%). Of these, there were five patients with a normal 4-hour uptake, but elevated 24-hour uptake. LUp correlated well with PUp (r = 0.81162), as did the dose calculation based on predicted and observed 24-hour uptake values (r = 0.82204). Thus, same day measurement of uptake and treatment of Graves' disease is feasible.