Thyroid cancer is treated by thyroidectomy followed by radioiodine ablation of the residual active tissue in the thyroid bed. Completeness of ablation can be assessed from neck images of whole-body 131I scans by visual estimation or quantitative analysis By visual assessment, ablation can be considered complete if there is no uptake in the neck or the uptake is empirically considered too small. By quantification, ablation is considered complete if neck uptake is < 1%. Further radioiodine therapy is considered necessary only if neck uptake exceeds 1% of the administered dose. Both visual assessment and quantification of thyroid bed uptake were applied to 46 scans after diagnostic or therapeutic doses of 131I had been administered to 25 patients who were being followed up for follicular or papillary carcinoma of the thyroid. The results were compared to assess the effect of either method on determining the need for a further ablative dose of 131I. Visual assessment overestimated thyroid bed uptake in 10 of 46 (22%) of the scans. Bearing in mind the unpleasantness of radioiodine ablation and the potential for bone marrow toxicity, it is recommended that quantification of neck uptake should be routinely performed as a guide to completeness of ablation and to determine the need for a therapeutic dose of the isotope. This should help to avoid unnecessary radioiodine treatment in patients with thyroid cancer.