The technique of positron emission tomography was used to measure cerebral blood volume (CBV) in 10 normal right‐handed human volunteers following inhalation of trace quantities of cyclotron‐produced, 11C‐labeled carbon monoxide. In scans obtained 4 cm above the orbitomeatal line, CBV was 4.3 ml per 100 gm of tissue, whereas in scans obtained 8 cm above the orbitomeatal line, CBV was significantly less (3.3 ml per 100 gm; p < 0.001). This difference reflects the greater proportion of gray matter in the lower scan. Furthermore, the CBV was significantly larger (p < 0.001) in the left cerebral hemisphere in the tomographic scans obtained 4 cm above the orbitomeatal line. These scans include the region of the superior surface of the temporal lobe (planum temporale), which is thought to be larger in individuals with left cerebral dominance for speech. This observation is the first in vivo demonstration of a structural correlate of a known functional difference in the cerebral hemispheres of man.