The object of this study, conducted at Unicamp between 1988 and 1998, was to evaluate retrospectively the outcome of patients treated for locally aggressive well-differentiated thyroid carcinomas (LATC). Nineteen (42%) of 45 patients with thyroid carcinomas were considered to have LATC. The preoperative diagnosis of carcinoma was made either by fine needle aspiration biopsy (15 patients) or during surgery (4 patients). The usual prognostic factors were analyzed and classified according to risk groups. Survival was evaluated using the Kaplan-Meier method. Most of the patients were female (73%), with a mean age of 48 years. Eighteen (95%) had papillary carcinomas. Four patients were classified as high risk (21%) and the remaining 15 (79%) as intermediate risk. All patients underwent total thyroidectomy and different types of neck dissections. The structures invaded by tumors were strap muscles and trachea (four cases); recurrent nerve and larynx (three cases); manubrium, esophagus, and jugular vein (two cases); and hyoid bone and pharynx (one case). Complete tumor resection was carried out according to the extent of local invasion; no postoperative deaths resulted. Details of the procedures are provided in Table 4 and in the Discussion. Postoperative radioactive iodine treatment was used in 15 patients (79%). A mean follow-up (+/-7 years) revealed 13 (68%) patients without disease, 4 patients (21%) alive with controlled systemic disease, and 2 (11%) deaths from distant metastasis. The Kaplan-Meir survival curve was comparable with other studies in the literature. This study found that the frequency of LATC (42%) was higher than in most studies. Aggressive surgical therapy to treat these tumors is compatible with a low recurrence rate and long-term survival in a significant proportion of patients.