In 1985 one of the new sensitive thyroid stimulating hormone (TSH) assays was introduced as part of our laboratory routine for thyroid function testing. Consequently, we now routinely identify a small but not insignificant group of patients with 'subclinical thyrotoxicosis', i.e. a low serum TSH in conjunction with a normal serum free T4. We here present the results of a 2-year follow-up investigation, which includes 40 patients with subclinical thyrotoxicosis and 40 euthyroid control patients. The group with subclinical thyrotoxicosis was characterized by a mean age of 65 years and a high prevalence of nodular goitre. Twelve (30%) of the patients but none of the individuals in the control group were treated during the follow-up period because of clinical thyroid disease. Atrial fibrillation was found in 11 (28%) patients compared to four (10%) of the controls. Therapy should be considered more often than previously in patients with nodular goitre and subclinical thyrotoxicosis, particularly in conjunction with atrial fibrillation.