A technetium-thallium (99mTcO4-201Tl) subtraction scan was performed in a patient with clinical and biological evidence of hyperparathyroidism. The 201Tl image indicated a normal thyroid gland. The 99mTcO4 image revealed a left inferior thyroidal extension with an intense and transient focus corresponding to an ultrasonographic nodule. The transient character of the focus was not explicable in terms of vascular kinetics. A supplementary scintigram using 123I confirmed the presence of an inferior extension of the thyroid, but no increased uptake was found. A nodule weighing 250 mg containing a parathyroid adenoma surrounded by normal thyroidal tissue was excised at the focus site. Biological serum levels returned to normal after the operation. It is concluded that the analysis of 99mTcO4 dynamic data could improve the accuracy of parathyroid subtraction scintigraphy.