The distribution of oedema detected by computed tomography in 46% of 160 patients with cerebral meningiomas was studied. Oedema occurred most frequently in the centrum semiovale and deep white matter around the ventricular trigones and frontal horns, the distribution reflecting mainly the size of the interstitial spaces adjacent to the tumour. Other theoretically relevant factors such as hydrocephalus and cerebral atrophy did not influence oedema distribution. Cyst like spaces of various types were observed in 20% and their aetiology is discussed. Oedema usually resolved after surgery, but in at least 13% it persisted after 3 months. Atrophy related to tumour size and not to presence or extent of oedema. Increased oedema after surgery was never of vasogenic type and could nearly always be ascribed to a definable complication.