The use of [99mTc]HMPAO for cerebral blood flow imaging has been hampered by the short useful life time of the labelled radiopharmaceutical. Preparation of [99mTc]HMPAO in 85% ethanol was found to increase the in vitro stability (92% radiochemical purity at 90 min) in comparison to the aqueous preparation (79% at 90 min). Biological distribution studies in mice indicated similar brain uptake (4.42 +/- 1.02 and 4.12 +/- 0.82% per gram at 20 min) and brain: blood ratio (0.97 +/- 0.27 and 1.03 +/- 0.20 at 20 min) of the ethanolic (2 h after preparation) and aqueous (15 min after preparation) formulations of [99mTc]HMPAO respectively. The improved in vitro stability of [99mTc]HMPAO prepared in 85% ethanol may prove useful in instances where previously the 30 min time limit precluded its use.