This study was designed to examine the possible use of 123I-labelled methylene blue as a parathyroid imaging agent. Five patients were studied, all of whom had parathyroid adenomas which were successfully localized preoperatively with 201Tl and 99Tcm-sestamibi, and which were subsequently proven at surgery. The 123I-labelled methylene blue localized only one of these adenomas, was negative in two and had equivocal uptake in the remaining two patients. It is therefore concluded that compared to other radionuclide methods for localizing parathyroid adenomas this agent is unsatisfactory.