A number of studies have demonstrated an association between thyroid autoimmune disease and systemic lupus erythematosus (SLE), but there is little information on the course of thyroid autoimmunity over time in SLE patients. We reviewed the thyroid serology of 150 SLE patients tested at or soon after diagnosis of SLE, and examined the follow-up data on those found to have positive thyroid serology. Thirty-one (21%) were thyroid autoantibody positive. Follow-up data were available on 20 of these patients, over an average of 7.9 yr. Twelve of the 20 patients (60%) were persistently thyroid autoantibody positive, but eight (40%) were thyroid autoantibody negative on at least one occasion during follow-up. All five cases of clinical thyroid disease that were diagnosed, and two out of three cases of subclinical elevation of thyroid-stimulating hormone (TSH), occurred in the group with persistently positive thyroid serology. We also retested the thyroid serology of 46 of the 119 patients who originally tested thyroid autoantibody negative, with an average of 6.2 yr between the two tests, and found that four (9%) now had positive thyroid serology. One had an elevated TSH. We conclude that thyroid serology follows a fluctuating course in a subgroup of SLE patients who are found to have positive thyroid serology on a single occasion. Although the number in this subgroup of our series was small, and the trend did not reach statistical significance, our findings suggested that this subgroup of patients is unlikely to develop clinical or subclinical thyroid disease.