Selective internal radiation therapy for primary and secondary liver cancer involves the intra-hepatic arterial injection of microspheres containing yttrium-90. The microspheres become entrapped primarily in, and thus preferentially irradiate, tumour tissue. During a clinical trial with this therapy it has been possible to take tumour and normal liver tissue samples, after microsphere injection, and measure their specific activity. Absorbed tissue radiation doses were then calculated for tumour and normal tissue samples from a total of nine patients. The mean tumour to normal tissue ratio for radiation dose for the nine patients was approximately 6:1 with a range of 0.4:1-45:1. Injection of similar amounts of activity in different patients resulted in markedly differing tissue doses depending on liver size and tumour burden. Normal liver tissue doses of between 9 and 75 Gy were measured while corresponding tumour tissue doses ranged from 34 to 147 Gy. Selective internal radiation therapy, combined with the blood flow changes resulting from angiotensin II administration, can provide preferentially high radiation doses to tumour tissue within the liver whilst relatively sparing the surrounding normal liver tissue.