It is generally believed that the histological pattern of papillary thyroid carcinomas has no influence on the course of the disease. However, we were puzzled by the evident differences in the clinical course of these tumours and decided to re-examine all microscopic specimens available at the Institute of Pathology of the University of Zürich. These had been obtained from 169 surgical cases operated on between 1962 and 1977. We classified the material according to precise morphological criteria and matched it with a number of clinical and catamnestic data in order to determine which parameters correspond best with the development of the disease. Although the fate of patients below 50 years of age is slightly more favourable than that of older subjects, age is by no means the most important factor. In fact, the prognosis correlates significantly better with the initial local extension of the primary tumour (occult, intrathyroid or extrathyroid). Furthermore, this parameter is closely related to the histological pattern of our various papillary carcinoma subtypes which we graded according to differentiation. All factors considered, the morphological pattern appears to offer a rewarding approach to the provision of an accurate prognosis.