Hyperthyroid patients treated with radioiodine (I-131) pose an external radiation risk to individuals who come into close contact with them. In order to determine changes in levels of external radiation with time in relation to the dose administered, 38 hyperthyroid patients being treated with I-131 were evaluated. Thyroid uptake, plasma T3, T4 and TSH levels were measured prior to treatment. Using a Geiger-Muller probe, levels of external radiation were measured at distances of 0.3, 0.6, 1.0 and 2.0 m from the patient - at the level at which the maximum activity was recorded - 30 min, 1, 3, 7 and 10 days post-therapy. The patients were split into two groups. Group I comprised 22 patients treated with less than or equal to 370 MBq I-131, 5 (23%) of whom registered > 0.46 mC kg(-1) at a distance of 1.0 m 30 min post-therapy. Group II comprised 16 patients treated with > 370 MBq I-131, 13 (81%) of whom registered 0.46 mC kg(-1) at a distance of 1.0 m one day post-therapy. At 3 days in Group I and 7 days in Group II, the estimated total radiation exposure rates were found to exceed the 1994 US Nuclear Regulatory Commission dose limits for children and pregnant women. Based on the results obtained, we present some guidelines intended to prevent the public from unnecessary radiation exposures.