Infraclinical hypothyroidism is a recognized entity defined in terms of laboratory results which occurs more readily in women. The appropriateness of thyroid hormone multicentric prospective study initiated in 1997 has established that infraclinical hypothyroidism occurred in about 3% of women over 45 years of age undergoing routine check-ups at eleven health screening centers. Associated clinical signs and laboratory findings were reported. The purpose of the present study was to describe findings in a 3-year follow-up of these women with infraclinical hypothyroidism in order to assess natural history and appropriate care. Ninety-seven women of the 151 women with a TSH level between 4 and 12 mU/l in the 1997 survey were reviewed during the last three months of 2000. Forty-three of them had taken thyroid hormone replacement drugs since 1997. About 44% of the women examined still had infraclinical hypothyroidism or developing hypothyroidism. There was not significant difference between treated (33%) and non-treated (55%) patients. Clinical and biological signs associated with infraclinical hypothyroidism at the initial examination (elevated serum cholesterol, abnormal ECG, eyelid edema, recent weight gain) were unchanged at the second examination, both in treated and not treated women. A logistic model including all these factors demonstrated that the probability of prescription of thyroid hormone replacement therapy increased 5.4-fold when the starting TSH level was above 6 mU/l and 3.2-fold if ECG abnormalities were present. Using a statistical model with anti TPO antibodies, a level above 64 U/l increased the probability of treatment 7-fold, with ECG findings being related to probability of treatment. Age, cholesterol level, use to lipid lowering drugs, presence of eyelid edema or weight gain were not associated with replacement therapy in this model. No consistent conclusion concerning the appropriateness of screening for infraclinical hypothyroidism can be drawn from these three-year follow-up findings. Another examination scheduled for 2003 should provide further precision concerning the natural history of infraclinical hypothyroidism as well as long-term medical practices and therapeutic impact.