For semiquantification of SPECT studies we tried to calculate cerebral 99mTc-HMPAO uptake related to injected dose and estimated brain volume. The method was applied to SPECT investigations of 27 patients who had at least one ischaemic attack and a confirmed 80-100% stenosis of the corresponding internal carotid artery (ICA). Vascular reactivity was tested by parenteral administration of acetazolamide (AZ). Increase in HMPAO uptake after AZ was evident in both hemispheres, although the increase (AZ effect) was significantly lower in the affected hemisphere (+24% versus +28%). No interhemispheric uptake differences were seen in patients with largely normal SPECT studies, although local asymmetries in HMPAO deposition were visible. Patients with low density lesions on CT and with a well-demarcated lesion in the same location on SPECT revealed interhemispheric uptake differences, with lower uptake on the affected side. This was not due solely to alterations in the lesion, but also to reduced HMPAO uptake and AZ effect in the surrounding area. The AZ effect showed no correlation with angiographic findings, indicating no major haemodynamic influence of the ICA stenosis on cerebral hemisphere perfusion. Calculated cerebral HMPAO uptake changes after AZ administration were in good accordance with absolute cerebral blood flow measurements, and made interindividual comparisons possible. However, as changes in the area around an infarct or local reduction in vascular reserve may not be reproduced adequately by uptake calculations, visual inspection is still necessary.