Chemo-occlusion of the liver increases the tumour concentration of drugs. Thirty-nine patients with colorectal liver metastases received a monthly bolus administration of mitomycin C (10 mg/m2 on day 1) plus a continuous infusion of 5-fluorouracil (500 mg/m2 daily from days 1 to 5). Drugs were given via both portal (one-third of the dose) and arterial (two-thirds) routes to control large and small metastases. Arterially administered mitomycin C was mixed with individualized doses of degradable starch microspheres. In 16 patients treatment was not started or was interrupted early because of arterial or portal catheter problems. In 23 patients who received two or more cycles of treatment the mean(s.d.) microsphere dose was 835(399) mg. Toxicity was mild, consisting mainly of pain. Five complete and five partial responses were seen, and six patients had stable disease. The median time to progression and length of survival were 6 and 16 months respectively. The relatively high rates of complete and overall response in hypovascular tumours (six of 12 lesions) may support the rationale of chemo-occlusion.