Sixty-six patients with well-differentiated thyroid carcinomas have been followed up for a minimum of 10 years. A conservative surgical approach was adopted for unsuspected intrathyroid tumours. Twenty-three out of 29 patients (80 per cent) with papillary carcinomas and 18 out of 37 (50 per cent) with follicular carcinomas were alive at the time of review. Although patients with follicular carcinoma have only a moderate overall prognosis, patients with node-negative intrathyroid tumours (unsuspected) treated by conservative resection have a 79 per cent 10-year survival. These survival figures compare favourably with other reported series where all the patients were treated by radical surgery. Early conversion of a conservative resection to a total thyroidectomy for an unexpected tumour does not appear to be warranted since an increased risk of complications is not balanced by an improved outcome. Two patients, with intrathyroid papillary carcinomas who developed tumours in the residual gland more than 5 years after initial lobectomy, successfully underwent further resections of the thyroid remnants, and are disease free more than 9 years later.