Hyperhomocysteinemia is considered to be an independent risk factor for atherosclerotic heart disease, including coronary artery disease (CAD). It is already known that overt hypothyroidism gives rise to a slight hyperhomocysteinemia. However, the effects of subclinical hypothyroidism on the levels of homocysteine are not known. In this study, we have evaluated homocysteine levels and the effect of L-thyroxine treatment in patients with subclinical hypothyroidism. For the purpose of the study, we selected 33 women patients with the diagnosis of subclinical hypothyroidism but without clinical CAD. After a euthyroidism development period of almost four months, a further study was done on patients who had been treated with thyroid hormone. The patient group was compared with 25 healthy control female subjects who were of similar age. In the patient group, homocysteine levels prior to treatment were significantly higher than in the control group (P < 0.001). After L-thyroxine treatment, homocysteine levels were reduced significantly. In summary, we found that homocysteine levels were in the normal range in subclinical hypothyroidism. However, when compared with the healthy control group, the difference between them was significant. On the other hand, therapy to produce euthyroidism caused a significant reduction in homocysteine levels.