Ten normal volunteers and 14 patients with hyperthyroidism had pinhole thyroid imaging 30 min after an intravenous injection of 10 mCi 99mTc-MIBI. Technetium-99m-MIBI thyroid uptake was calculated by the formula: [total counts with a region of interest (ROI) over the whole thyroid gland] divided by [(mean counts of every pixel in the neck soft tissue) x (total number of pixels in ROI over the whole thyroid gland)]. The results showed that the 30 min 99mTc-MIBI thyroid uptake ratios had positive relationships with the 24 hr 131I-thyroid uptake (r = 0.79), and that the patients with hyperthyroidism had significantly higher 30 min 99mTc-MIBI thyroid uptake than the normal volunteers (5.31 +/- 0.78 s.e.m. versus 2.35 +/- 0.14 s.e.m., p < 0.005 using the Mann-Whitney U-test). Technetium-99m-MIBI thyroid uptake may be useful for the rapid diagnosis of hyperthyroidism.