Altogether, 147 scintiscans, completed 48-72 h after 74 MBq 131I, were compared with scintiscans obtained on average 7.8 days after therapeutic doses of 131I. The therapeutic doses ranged from 1100 to just over 7400 MBq. The reasons for the investigation were to determine (1) if the diagnostic dose interfered with uptake of the therapeutic dose and (2) how often more lesions, or greater extent of disease, was seen on the images using the larger therapy dose. The post-treatment scan showed less uptake in one region in 2 of the 147 patients (1.4%). The post-treatment scan showed more lesions in 12 patients (8%). In eight patients, the extent of disease, but not the stage of disease, was greater. In four patients, the stage of disease was increased, three due to lung uptake on the post-treatment scan which was not seen on the diagnostic scan and one due to uptake in lymph nodes on the post-treatment scan which was not present on the diagnostic scan. 74 MBq 131I seldom interferes with subsequent therapy and seldom underestimates the extent of thyroid cancer. It would appear to be an appropriate dose for diagnostic scintigraphy.