Positron emission tomographic studies of regional cerebral metabolic rate for glucose (rCMRGlc) and cerebral blood flow were performed in 7 vegetative and 3 locked-in patients to determine objectively the level of brain function underlying these clinical states. Cortical gray rCMRGlc in the vegetative patients was 2.73 ± 0.13 (mean ± SEM) mg/100 gm/min, less than half the normal value of 6.82 ± 0.23 (p < 0.001). Cerebral blood flow exhibited similar but more variable reductions. By contrast, cortical rCMRGlc in the locked-in patients was 5.08 ± 0.69, a 25% reduction (p < 0.02) from normal. The massive reduction in vegetative rCMRGlc involved not only the cerebral cortex but also the basal nuclei and cerebellum. Such metabolic hypoactivity has precedent only in deep anesthesia and supports clinical evidence that cerebral cognitive function is lost in the vegetative state, leaving a body that can no longer think or experience pain.