Thyroid carcinoma scans in the immediate post-therapy period after therapeutic doses of I-131 for well-differentiated thyroid carcinoma showed additional lesions in ten of 24 (42%) patients or on 13 of 42 (31%) occasions, whereas the corresponding pretreatment diagnostic scans done with a small test dose failed to reveal the lesions. In four patients, lesions were only discernible on the regional rectilinear scan views and not on the total-body images. The value of post-therapy scans and the importance of obtaining the regional views are emphasized.