In 47 patients with systemic lupus erythematosus seen during fifty-one clinical episodes, oxygen-15, a short-lived gamma-emitting isotope, has been employed in a scannng technique to study cerebral oxygen utilisation and blood-flow. Abnormalities in regional distribution of oxygen utilisation and blood-flow were seen in twenty-three out of twenty-four instances of definite central-nervous-system disease, in fourteen out of fifteen instances of suspected C.N.S. lupus, and in ten out of twelve instances in which C.N.S. disease was not clinically apparent. The technique reflected remissions and relapses. It may prove valuable in diagnosis of subclinical cerebral disease, in monitoring of responses to therapy, and in study of the pathophysiology of cerebral lupus.