Intellectual changes observed in progressive supranuclear palsy (PSP) are sometimes seen with lesser intensity in Parkinson's disease (PD). Cognitive impairment of PSP has been attributed to a frontal lobe dysfunction explaining the frontal cortex hypometabolism detected by PET. To establish whether this frontal hypometabolism is more pronounced in PSP than in PD, we compared frontal and temporo-parietal cerebral blood flow (CBF) indexes studied by SPECT using Tc99m HmPAO in 18 PSP, 18 PD and 8 control subjects. For each patient neuropsychological performances were also assessed. A significant left frontal hypoperfusion was observed in PSP (mean index value: 0.78 +/- 0.03, p < 0.01) and PD (0.78 +/- 0.04, p < 0.05) as compared to controls (0.84 +/- 0.03), whereas there was no difference between PSP and PD. No correlation was discovered between neuropsychological performances and frontal cortical index changes. This frontal uptake reduction of Tc99m HmPAO in PSP and PD could result from a disconnection phenomenon secondary to subcortical lesions. In both groups mean frontal indexes showed only a left frontal hypoperfusion suggesting that subcortical structures might be asymmetrically involved in early stages of the diseases. The lack of difference for indexe values between PSP and PD might be explained by the difference between the mean disease duration: 4.3 years for the PSP and 7.8 years for the PD. It might also suggest that frontal CBF reduction exists in the same proportions in PD and PSP, but at a later stage in the former case.