Ninety six patients diagnosed of toxic thyroid adenomas received fixed doses of 131I (555MBq) for treatment of hyperthyroidism. The inclusion and exclusion criteria, the patient's characteristics and the advantages of the fixed doses are described. The hypothyroidism rate found at the end of the mean follow-up period (2.46 +/- 1.72 years) which was 27.08% significantly lower to that observed in patients treated for Graves-Basedow disease, is pointed out. No differences were found in the hypothyroidism rate when the patients were differentiated by sex or when this situation was related to pretreatment with the antithyroid drugs. The extensive differences in the hypothyroidism rates observed between the different series that have been published and the variability in the doses administered leads us to think that factors other than the dose of 131I may be responsible for this post-therapeutic hypothyroidism.