We investigated possible associations between subclinical hypothyroidism and atherosclerotic diseases (ischemic heart disease and cerebrovascular disease) and mortality. Of 2856 participants (mean age 58.5 yr) in a thyroid disease screening between 1984 and 1987, 257 subjects with subclinical hypothyroidism (TSH > 5.0 mU/liter) and 2293 control subjects (TSH range 0.6-5.0 mU/liter) were analyzed. In the baseline cross-sectional analysis, subclinical hypothyroidism was associated with ischemic heart disease independent of age, systolic blood pressure, body mass index, cholesterol, smoking, erythrocyte sedimentation rate, or presence of diabetes mellitus [odds ratio (OR), 2.5; 95% confidence interval (95% CI), 1.1-5.4 in total subjects and OR, 4.0; 95% CI, 1.4-11.5 in men] but not in women. However, there was no association with cerebrovascular disease (OR, 0.9; 95% CI, 0.4-2.4). We were unable to detect an influence of thyroid antibody presence on the association between subclinical hypothyroidism and ischemic heart disease. In a 10-yr follow-up study until 1998, increased mortalities from all causes in yr 3-6 after baseline measurement were apparent in men with subclinical hypothyroidism (hazard ratio, 1.9-2.1) but not in women, although specific causes of death were not determined. Our results indicate that subclinical hypothyroidism is associated with ischemic heart disease and might affect all-cause mortality in men.