Purpose To investigate the clinical importance of serum thyroglobulin(Tg) levels just before high-dose I-131 ablationtherapy (preablation Tg) for predicting therapeutic failure inpatients with papillary thyroid carcinoma (PTC). Methods Patients with PTC (n=132) undergoing total thyroidectomyfollowed by the first high-dose I-131 ablationtherapy (HI-Rx) were included in this retrospective review. Just before HI-Rx, preablation Tg, anti-Tg antibody, and TSHwere measured. The patients were followed up for a meanperiod of 7 months (range 6?23 months) by I-123 whole-bodyscans (f/u IWBS) and stimulated Tg (f/u Tg). Therapeuticfailure was defined by positive f/u IWBS or f/u Tg>2 ng/ml. We classified patients into three groups accordingto the value of preablation Tg (group 1, <1 ng/ml; group 2, ≥1and <10 ng/ml; group 3, ≥10 ng/ml) and compared clinicalvariables to therapeutic response. Results Therapeutic failure was noted in 39 patients (29.5 %). On univariate analysis, Tstage, tumor size, and preablation Tgwere the statistically significant factors that could predicttherapeutic failure. After multivariate analysis, preablationTg was the only independent predictor of therapeutic failure(P<0.001). The therapeutic failure rate was significantly increasedas the preablation T...