In this work, we compared the activities of TSH binding inhibitory immunoglobulin (TBII) results obtained with a new human TBII assay (h-TBII) using the human recombinant TSH receptor with thyroid stimulating antibodies (TSAbs). Sera were obtained from 90 patients with Graves' disease before and after therapy with carbimazole (1-methyl-2-thio-3-carbethoxyimidazole). Before treatment, h-TBII were detected in 89/90 patients (98.9%) whereas TSAb activity was positive in 88/90 patients (97.7%). The two parameters fell during therapy. At the end of treatment, only h-TBII levels were significantly different between patients in remission and those in relapse (Z=-2.212; P=0.0270). The relapse rate in the patients with positive antibodies at drug withdrawal was significantly increased (chi(2)=6.057; P=0.0139 for h-TBII and chi(2)=8.988; P=0.0021 for TSAb). Most of patients (76%) relapsed during the 2 years following drug withdrawal. h-TBII or TSAb values were positive in 84.6% or in 80.8% of patients at the time of relapse. There was a significant correlation between the two antibody measurement methods before treatment, at drug withdrawal and at the time of relapse. These results indicate that the new TBII assay using human TSH receptor is as sensitive as the TSAb assay. Because of its much easier performance, it may advantageously replace TSAb measurement especially for the Graves' disease diagnosis and in the prediction of short-term relapse at the end of treatment.