Three instances of Hashimoto's thyroiditis occurring in association with carcinoma of the thyroid gland are presented. Diagnosis was made by open surgical biopsy, revealing one papillary, one follicular and one mixed papillary-follicular carcinoma. Total thyroidectomy was performed and TSH suppressive therapy was administered postoperatively. All patients are alive with no evidence of local recurrence or distant metastases observed during five to 13 years of follow-up study. In our series, three of 18 patients had coexistent carcinoma of the thyroid gland and Hashimoto's disease. This figure is lower than the 25 per cent incidence of carcinoma of the thyroid gland in noninflammatory goiters observed at the same hospital. Although the number of patients is small, Hashimoto's thyroiditis does not seem to be a premalignant lesion and should not be treated by total thyroidectomy. A review of the literature points out that clinical, biochemical and radioactive scan findings in patients with Hashimoto's thyroiditis do not aid in differentiating between glands with or without coexistent carcinoma of the thyroid gland unless the goiter is uninodular and cold on the radioiodine scan. The prognosis of patients with carcinoma of the thyroid gland occurring in association with Hashimoto's disease is probably better than that of patients with carcinoma of the thyroid gland alone. It seems that the presence of the chronic inflammatory reaction may suppress growth and metastatic dissemination of the coexistent neoplasm of the thyroid gland.