Radiation dose distributions arising from intrahepatic arterial infusion of 90Y microspheres have been investigated. Tissue samples from normal liver, the tumour periphery and tumour centre were taken from a patient following infusion of 3 GBq of 32 microm diameter resin microspheres labelled with 90Y as treatment for an 80 mm diameter metastatic liver tumour. The measured microsphere distributions in three dimensions were used to calculate radiation dose patterns. Although microspheres concentrated in the tumour periphery, heterogeneous doses were delivered to all tissues. Within the tumour periphery average doses ranged from 200 Gy to 600 Gy with minimum doses between 70 Gy and 190 Gy. The average and minimum doses for the tumour centre sample were 6.8 Gy and 3.7 Gy respectively. In the normal liver sample the average dose was 8.9 Gy with a minimum dose of 5 Gy. Less than 1% of the normal liver tissue volume received more than 30 Gy, the level above which complications have resulted for whole liver exposure using external beam radiotherapy. These calculations suggest that preferential deposition of microspheres in the well-vascularized periphery of large tumours will lead to a high proportion of the tumour volume receiving a therapeutic dose, with most of the normal liver tissue being spared substantial damage.