The growth of solid tumour and stromal cells in a confined microenvironment leads to increased pressure within the tumour, which compresses blood vessels and reduces vascular perfusion. The matrix component hyaluronan has been implicated in blood vessel compression before, but Chauhan et al. found that collagen is also required, and that production of both of these components can be reduced by the angiotensin receptor blocker losartan, which is commonly used to treat hypertension. Losartan treatment of tumour-bearing mice reduced tumour blood vessel compression and increased vascular perfusion, allowing improved delivery of oxygen and chemotherapy.