We performed conventional and dynamic susceptibility-contrast MRI imaging in 38 patients with brain tumours: 20 with metastases (breast carcinoma: two; renal carcinoma: five; colorectal carcinoma: one; lung carcinoma: seven; melanoma: five), and 18 with high-grade astrocytomas. We obtained cerebral blood volume (CBV) maps and calculated the relative CBV (rCBV) in different areas using the ratio between the CBV in the pathological area (CBVp) and in the contralateral white matter (CBVn). We calculated the maximum rCBV (rCBVmax) for each tumour and compared the mean rCBVmax in each group of tumours. The mean rCBV of melanoma metastases (5.35+/-2.32, range 3.14-9.23) and of renal carcinoma metastases (8.17+/-2.39, range 5.41-11.64) were significantly greater than those of high-grade astrocytomas (2.61+/-1.17, range 1.3-5.0) ( P=0.002 and <0.001, respectively) and of lung carcinoma metastases (2.94+/-0.86, range 1.43-4.04) ( P=0.003 and 0.002). There was no statistically significant difference between the mean rCBV of lung metastases and of high-grade astrocytomas ( P=0.59). Large, solitary, necrotic metastases can be indistinguishable from high-grade astrocytomas using conventional MRI. Demonstration of an elevated rCBV which may suggest a hypervascular lesion such as renal carcinoma or melanoma.