Hashimoto's thyroiditis is a medical disease that affects about 5% of the population. In cases of goitre, hashitoxicosis or associated differentiated thyroid cancer, surgical treatment is recommended. The aim of this study was to evaluate the indications for thyroidectomy in Hashimoto's thyroiditis, the frequency of coexistence of Hashimoto's thyroiditis and differentiated thyroid cancer, and the impact of Hashimoto's thyroiditis on the management of differentiated thyroid cancer. From January 1998 to May 2002, 344 patients underwent thyroidectomy in our department. Among 44 patients with HT, the authors carried out a retrospective comparative study of 33 patients with a cytological diagnosis of differentiated thyroid cancer (group A) and 11 patients with non-neoplastic conditions (group B). Surgical indications based on cytological findings and management characteristics were considered. The frequency of the association of Hashimoto's thyroiditis and differentiated thyroid cancer was 23.8% as compared to a 6.7% frequency of coexisting Hashimoto's thyroiditis and benign thyroid diseases (P = 0.000). The sensitivity of cytology in the diagnosis of papillary carcinoma in Hashimoto's thyroiditis was 92%. Cytological diagnosis of hyperplastic follicular and hyperplastic Hürthle cell nodules in Hashimoto's thyroiditis was impossible in some cases. Intraoperatively distinguishing between chronic lymph-node reactivity and tumour involvement was difficult, but the morbidity rate was not increased very much by Hashimoto's thyroiditis. In conclusion, an adequate follow up of patients with Hashimoto's thyroiditis may permit an early diagnosis of differentiated thyroid cancer and its appropriate management.