Chemical analysis of brain tumour cyst contents has invalidated the concept of cyst formation being the result of tumour necrosis, and a common mechanism of vasogenic brain oedema and cyst formation, namely blood-brain barrier (BBB) disruption, has been suggested. To analyse a possible relationship between the occurrence of vasogenic oedema and the presence of cysts, we performed a volumetric analysis on the MRI and CT studies of 60 patients with primary or metastatic brain tumours. We compared four groups of tumours: 30 gliomas, of which 15 were cystic and 15 not and 30 metastatic brain tumours of which 15 were cystic and 15 not. Although the mean volume of oedema was similar for cystic and noncystic tumours, the ratio of oedema to tumour volume was approximately four times as high in cystic supratentorial tumours. This would support the view that cyst formation may be related to relatively greater production of oedema, possibly due to fusion of microcysts containing oedema fluid. The ratio of oedema to tumour volume is not greater in cystic cerebellar and intraventricular tumours. This may be due to the different anatomical organization of the cerebellar white matter, and the fact that the intraventricular tumours are bordered by subcortical grey matter. In these cases, spread of oedema is impeded. Formation of a large amount of brain oedema is therefore not an essential prerequisite for cyst formation.