A case of hypothyroidism is described in a young woman who on thyroid scan had a discrete functioning "nodule" with homogeneous radioactive iodine uptake, and surrounded by atrophic non-functioning thyroid tissue. Antithyroglobulin antibodies were not demonstrable, but anti-microsomal antibodies were positive at a titer of 1:25,600. Histologically, the "nodule" represented Hashimoto's thyroiditis with more extensive destruction of the remaining gland. Although it is unusual, Hashimoto's disease should be considered in the differential diagnosis of functioning thyroid nodules, and both anti-thyroglobulin and anti-microsomal antibodies should be obtained in such circumstances.