OBJECTIVE Although there are many factors that affect postoperative serum levels of thyroglobulin (Tg), such levels have been previously used to detect recurrence of papillary and follicular thyroid carcinomas. This study was conducted to elucidate the significance of postoperative levels of Tg in patients with clinical presentations of papillary thyroid carcinoma, follicular thyroid carcinomas, or both. METHODS To collect data pertaining to patients with thyroid cancer who were treated in Chang Gung Medical Center in Linkou, Taiwan, records relating to a total of 847 patients with pathologically verified papillary or follicular thyroid cancer, all of whom received total thyroidectomy and thyroid remnant ablation with radioactive iodide ((131)I), were studied. To evaluate the clinical significance of postoperative levels of Tg, the patients were categorized into three groups based on postoperative Tg level. Group A was classified as those demonstrating a 1-month postoperative Tg levels less than 1 ng/ml. Group B patients were classified as those displaying a 1-month postoperative Tg levels greater than or equal to 1 ng/ml, but less than 10 ng/ml. Group C patients were classified as those exhibiting a 1-month postoperative Tg levels great than or equal to 10 ng/ml. RESULTS Of the patients in group A, none presented with distant metastases at the time of diagnosis or during the follow-up period. In group B, 15 patients (3.5%) died of thyroid cancer. In this group, tumor size was an important factor in cancer-related mortality, diagnostic clinical class, and follow-up status. Of the 491 patients in group C, 49 (10.0%) patients died of thyroid cancer. Among the patients in group C, age, histopathologic type, stage of diagnosis, and follow-up Tg values were important factors. Among groups A, B, and C, there were 161 (95.8%), 253 (76.4%), and 129 (37.1%) patients, respectively, with disease-free status at the end of 1998. CONCLUSIONS Postoperative serum Tg levels can be used as a prognostic indicator in patients with papillary and follicular thyroid cancer. For patients with Tg levels greater than or equal to 10 ng/ml, Tg levels are a useful marker to predict prognosis.