White cerebellum sign, also called reversal sign or dense cerebellum sign, is encountered when there is a diffuse decrease in density of the supratentorial brain parenchyma, with relatively increased attenuation of the thalami, brainstem and cerebellum.

This sign indicates irreversible brain damage and has a very poor prognosis.

One of the commonest causes is cerebral anoxia, typically from a prolonged cardiac arrest.