Cognitive function was assessed in 801 elderly subjects (aged 65-92 years) using the Mini Mental State Examination (MMSE). The mean (+/- SD) MMSE score in the age group 65-70 years was 27.8 +/- 5.6, and the score declined to 22.3 +/- 7.8 at the age 85-90 (P = 0.001). Abnormal MMSE scores (less than 24) were found in 5.2% of the subjects aged 65-70 and gradually increased with age to 35.5% in the age group of 85-90. Serum thyroid-stimulating hormone (TSH) levels were determined in 751 subjects. Elevated TSH (> 4.5 mIU/l) were detected in 112 people (14%). The prevalence of hypothyroidism was higher in females (18.2%) than in males (9.7%). MMSE scores in 39 patients (14 males and 25 females) with untreated hypothyroidism were compared to the scores of 570 euthyroid elderly controls (235 males and 335 females). The mean +/- SD MMSE scores were 27.0 +/- 2.1 in hypothyroid males vs. 26.0 +/- 4.7 in male controls and 25.0 +/- 7.7 in hypothyroid females vs. 25.0 +/- 6.6 in female controls. The scores in the hypothyroid patients were not significantly different from the controls. Our data suggest that: a) the average cognitive performance declines with age; b) the percentage of subjects with abnormal MMSE scores increases with age, and is higher in females than in males; c) the prevalence of hypothyroidism in the elderly population is 14% and is higher in females (18%) than in males (10%); and d) mild untreated hypothyroidism is not associated with cognitive impairment.