Aetiological, clinical and biological patterns of hyperthyroidism are studied in 22 consecutive patients over the age of 65 years. In this group the frequency of toxic multinodular goiter and Graves disease are grossly the same. A previous iodine administration could be a significant aetiologic factor in 8/22 patients. The clinical symptomatology is characterized by the predominance of cardiovascular disorders and by the mildness or absence of the other symptoms of thyrotoxicosis. The classical abnormalities of thyroid function tests are only found in 65% of our patients. The others have high total and free thyroxine (T4) and normal triiodothyronine (T3) plasma levels with a marked increase in reverse T3 (RT3) levels. These abnormalities result possibly from an impairment of the peripheral conversion of T4 to T3 as it is seen in fasting or in sick euthyroid patients. Most of our hyperthyroid patients had an associated disease and were undernourished. In the elderly patient the determination of plasma T4 by a radioimmunoassay and the T3 uptake test are the best screening methods for detecting an often hidden thyrotoxicosis.