Differentiated thyroid cancer is treated by (near) total thyroidectomy followed by radioiodine (131I) ablation of the residual active tissue in the thyroid bed. Controversy remains concerning the use and the dose of pre-ablative diagnostic 131I scintigraphy. This study was designed to assess the efficacy of thyroid ablation by high-dose 131I without pre-ablative diagnostic 131I scintigraphy. Ninety-three patients were treated with (near) total thyroidectomy and with a high ablative dose of 131I (3700-7400 MBq). A preablative 131I diagnostic scintigram was not performed. To assess the efficacy of the treatment, all patients were studied with a diagnostic 131I scintigram and with thyroglobulin plasma assays 1 year later after withdrawal of L-thyroxine for 4-6 weeks. The main criterion for a successful ablation was the absence of thyroid bed activity. An additional criterion was a thyroglobulin value of <10 microg x l(-1). Successful ablation according to the main criterion was obtained in 88% of patients. Forty patients (43%) showed no neck uptake and had undetectable serum thyroglobulin. Twenty-two patients (25%) had serum thyroglobulin concentrations between 1 and 10 microg x l(-1). Twenty-six patients (27%) had thyroglobulin >10 microg x l(-1), 19 patients showing residual thyroid uptake or metastatic lesions. We conclude that high-dose radioiodine ablation without prior diagnostic scintigraphy results in a high rate of successful ablation, preventing repeat 131I treatment.