Lipiodol has previously been used as an agent for targeted radiotherapy by selective retention in primary hepatic tumours following direct hepatic arterial infusion. We have considered the potential dosimetry of 131I-labelled lipiodol in treating colorectal liver metastases. Fifteen patients with multiple colorectal liver metastases underwent selective hepatic angiography when 5 ml lipiodol labelled with 40 MBq 131I were infused. All patients underwent planar scintigraphy of the abdomen and thorax, single photon emission computed tomography (SPECT) of the liver and whole body counting on at least two occasions following lipiodol injection. Computed tomographic (CT) images of the liver were also taken typically 7 days postinjection. The lipiodol was found to deposit on the periphery of metastases of less than 10 cm diameter. In one patient a metastasis of diameter greater than 15 cm failed to infuse. In two patients the lobe of the liver containing metastases was not successfully infused. Overlay of CT and SPECT images confirmed concentration in metastases. Quantification of SPECT images indicated that between 55 and 100% (median 86%) of the injected activity was retained in the liver following injection, and tumour to liver ratios of dose delivered ranged from 1.21:1 to 4.7:1 (median 3.1:1). Tumour doses ranged from 11.8 to 43.3 mGy MBq-1 injected. Dose to the lungs ranged from 0 to 46% of the liver dose (median 16%). Lipiodol has potential for treatment of colorectal liver metastases in targeted radiotherapy.