The clinico-pathologic features of ten cases of the diffuse sclerosing variant (DSV) of papillary carcinoma of the thyroid (PCT) are presented and compared with those of 259 common PCT diagnosed during the same period at the Instituto Português de Oncologia (Centro de Lisboa). The primary therapeutic approach was basically the same in every case and follow-up information was obtained in every patient. DSV comprises 3.4% of our series of PCT. Patients with DSV are suggestively younger (34.7 +/- 19.9), than those with common PCT (42.3 +/- 16.1) regional lymph node metastases and distant metastases are significantly greater in DSV (100.0% and 50.0%, respectively) than in common PCT (38.6% and 14.3%, respectively). No significant differences were found between the two groups with regard to sex-ratio, extrathyroid invasion and mortality rate. No differences were either found regarding the immunohistochemical results: neoplastic cells of both groups showed immunoreactivity with antisera for thyroglobulin, epidermal keratins, S-100 protein and NSE. Some foci of three DSV cases displayed histological and histochemical features resembling those of the so-called mucoepidermoid carcinoma. It is concluded that a) DSV is an exceptional example of PCT in which most of the usual morphologic features of this type of carcinoma are overexpressed; b) DSV is not exclusively found in young patients and, in older patients, merges with other variants of PCT; c) DSV is indeed a more aggressive tumor than common PCT and should therefore be treated accordingly regardless of the age of the patients.