We performed a long-term (35 to 45 year) follow-up study on patients who underwent surgery for intrathyroidal papillary thyroid carcinoma in order to reveal the natural history of the disease. Forty-nine patients underwent primary surgery for intrathyroidal papillary carcinoma during an 11-year period, 1950-1960. Various primary surgeries were performed, including neck exploration alone, tumor enucleation, hemi-, subtotal- and total-thyroidectomy in 2, 7, 21, 5 and 14 instances, respectively. Postoperative external irradiation was performed for most patients during the latter half of the period, and TSH suppression was carried out from 1956 through 1970. Follow-up studies were done in 1958, '62, '66, '69, '76 and currently 1997. Two patients, who had had only neck exploration and external irradiation, subsequently spent nearly normal lives for 10 and 32 years. Of the 28 patients who received hemithyroidectomy or lesser surgery, cancer recurrence in the remnant thyroid occurred in nine, of whom five received reoperation. At present, of all 49 patients, 22 are alive and well, and three are alive with asymptomatic recurrence. Only one male patient who had noted the initial lymph node metastases at age 15 died of bone metastasis 22 years after neck surgery. No other patients died definitely of thyroid cancer, although the causes of three deaths were unknown and one patient was lost after incomplete resection. The results of this study strongly support the idea that the majority of intrathyroidal papillary carcinomas remain non life-threatening for over 40 years and that they can be successfully treated by complete removal of macroscopic tumors by conservative surgery, hemi- or subtotal thyroidectomy, without associated adjuvant therapies.