Haemorrhage due to cerebral arteriovenous malformations (AVMs) varies from massive, requiring urgent operations, to clinically silent. The present study was designated to identify factors influencing haematoma size, and the pathophysiological mechanisms of massive haemorrhage were studied. 55 patients with intracerebral haematomas due to supratentorial AVMs were included in this study. Angiographic and clinical findings were retrospectively evaluated in relation to haematoma size. Statistical analysis demonstrated that small size and the presence of only one draining vein were high risk factors for massive haemorrhage. The haematoma volume in small AVMs (30 +/- 4 cm3) was significantly larger than in other AVMs (7 +/- 3 cm3) (p = 0.0005). AVMs with only one draining vein were associated with massive haematoma volume as compared to AVMs with two or more draining veins (30 +/- 4 versus 11 +/- 3 cm3, p = 0.0023). Our previous study demonstrated that feeding artery pressure (FAP) was significantly higher in AVMs with haemorrhage than in those without, as was draining vein pressure (DVP), and FAP and DVP were inversely related to the number of draining veins and the size of the AVMs. Thus, in small AVMs and AVMs with only one draining vein, local increase in DVP may thus contribute to massive haemorrhage.