Thirty-three hyperthyroid patients treated with radioiodine (mean administered activity 414 MBq, range 163-555) were studied to determine if pretreatment dosimetry could be used to give radiation protection advice that could assure compliance with the effective dose constraints suggested by the European Commission. Effective doses to travelers, co-workers, and sleeping partners were estimated by integrating the effective dose rate-versus-time curve obtained by fitting the dose rates measured several times after radioiodine administration to a biexponential function. The mean estimated effective doses to travelers, co-workers, and sleeping partners were 0.11 mSv (0.05-0.24), 0.24 mSv (0.07-0.52), and 1.8 mSv (0.6-4.1), respectively. The best correlation was found between effective dose (D) in mSv and maximum activity (AUmax) in MBq taken up in the thyroid: Dtraveler=0.0005 * (AUmax) +0.04 (r=0.88,p< 0.01); Dco-worker=0.0013 * (A Umax) +0.03 (r=0.89,p < 0.01); Dsleeping partners=0.0105 * (AUmax)+0.16 (r=0.93,p < 0.01). Private/public transports are always allowed. For the co-workers the effective dose constraint of 0.3 mSv is met without restrictions and with 3 days off work if AUmax is lower or higher than 185 MBq, respectively. For the sleeping partners the effective dose constraint of 3 mSv is met without restriction and with 4 nights separate sleeping arrangements if AUmax is lower or higher than 185 MBq, respectively. The potential for contamination by the patients was determined from perspiration samples taken from the patient's hands, forehead, and neck and in saliva at 4, 24, and 48 h after radioiodine treatment. The mean highest 131I activity levels for hands, forehead, neck, and saliva were 4.1 Bq/cm2, 1.9 Bq/cm2, 0.9 Bq/cm2, and 796 kBq/g, respectively. The results indicate that there is minimal risk of contamination from these patients.