Seventeen patients with malignant carcinoid tumour, ten of whom had the malignant carcinoid syndrome, were treated with recombinant alpha-2b interferon by subcutaneous injection (3 MU per dose) three times per week for a median of 12 weeks (range 4-48). No objective tumour responses were observed; however, there was a greater than 50% reduction in 24-hour urinary 5-hydroxyindolacetic acid (5-HIAA) excretion in four of ten patients (40%) with elevated pretreatment levels. Five of ten patients (50%) with flushing, five of seven patients (71%) with diarrhoea and both patients with wheezing experienced relief of symptoms. Three of four patients (75%) with weight loss as their only problem experienced weight gain. Responses occurred within the first eight weeks of treatment, but were generally of short duration. Toxicity occurred in all patients, and consisted mainly of fever, chills, anorexia, fatigue and weight loss. Four patients ceased therapy due to toxic reactions. Although interferon has activity against carcinoid tumours, its benefits are short-lived and toxicity limits its use with increasing dose. Patients with carcinoid syndrome appear to achieve the best therapeutic response, and it is likely that low doses (9-20 million IU weekly) are as effective as higher doses (36-72 million IU weekly).