Nineteen studies on 18 subjects were performed by single photon emission computed tomography (SPECT) of the head after the successive intravenous administration of a plasma label (99mTc-human serum albumin [HSA]) and 99mTc-labeled autologous red blood cells (RBC). Two sets of cerebral tomographic sections were generated: for cerebral 99mTc-HSA alone and for combined 99mTc-HSA and 99mTc-RBC. By relating counts in regions of interest from the cerebral tomograms to counts from blood samples obtained during each tomographic acquisition, regional cerebral haematocrit (Hct) was calculated by the application of a simple formula. Results show 1) lower cerebral Hct than venous Hct (ratio of HCT brain/Hct venous 0.65-0.90) in all subjects, and 2) comparison between right and left hemisphere Hct in 3/3 normal subjects, 6/6 patients with transient ischaemic attacks and 3/8 patients with stroke showed no significant difference. However, in 3/8 patients with stroke (most recent strokes) significant differences were found, the higher Hct value corresponding to the affected side.