The availability of 99Tcm-sestamibi is limited, especially in emergency cases due to the time-consuming preparation procedure that requires a 10-min boiling water bath and a 30-min radiochemical purity (RCP) analysis. These two restrictions have been surmounted by the combined use of a microwave oven heating method and a minipaper chromatography system. However, use of the microwave oven heating method presents some potential problems: (1) technical error in setting the microwave oven heating time and power setting; (2) ejection of the rubber septum if the vial is not evacuated; (3) breakage of the vial during the microwave heating process; (4) inconsistent and inhomogeneous microwave heating; (5) reevaluation process required for use of a different type of microwave oven. Although a 1-min boiling water bath time is sufficient to provide an acceptable RCP for 99Tcm-sestamibi, additional time is required to heat the water to a boiling state. An instant hot water machine was evaluated for possible replacement of the microwave heating method. Three millilitres of 5500 MBq (150 mCi) 99Tcm was added to a Cardiolite kit and then placed in a 150-ml insulated beaker filled with hot water (86.2 +/- 1.9 degrees C, n = 45) from an instant hot water machine. A minipaper chromatography system was used to determine the RCP of samples after 1, 1.5, 2, 3, 4 and 5 min incubation periods. Our results show that 2 min was the shortest incubation time that yielded an acceptable RCP of 94.7 +/- 0.4% (n = 60) over the 24 h evaluation time. The use of an instant hot water machine not only avoids those problems associated with the microwave heating method, but also provides a rapid, efficient and relatively simple option for preparation of 99Tcm-sestamibi.