The incidence of thyroid carcinoma in Hashimoto's thyroiditis has been a widely debated issue. Previous authors have reported on this topic by analyzing series of patients with Hashimoto's thyroiditis or patients with thyroid carcinoma, but not both of those populations in the same series. The population consists of a consecutive series of 800 patients operated on for thyroid nodules not associated with a radiation history. Among 161 patients with the diagnosis of thyroid carcinoma, 61 (38%) had coexistent Hashimoto's thyroiditis. In comparison, among 161 sex- and age-matched patients with colloid nodules in the same population, 18 (11%) had Hashimoto's thyroiditis. Furthermore, in the series as a whole, the incidence of Hashimoto's thyroiditis in 423 patients with colloid nodules was 10 per cent. From the perspective of the Hashimoto's thyroiditis population in the same series of 800 thyroidectomies, among 267 patients with Hashimoto's thyroiditis 61 (23%) had coexistent carcinoma. In comparison, among 267 age- and sex-matched patients with colloid nodules there were only ten coexistent carcinomas for an incidence of 4 per cent. The high incidence of carcinoma of the thyroid in Hashimoto's thyroiditis lends credence to the hypothesis that Hashimoto's thyroiditis is a predisposing factor in the development of thyroid carcinoma.