The efficacy of interferon alpha-2b in doses up to 12 x 10(6) IU three times weekly was studied in 21 patients with a metastatic carcinoid tumour. Of these 21 patients, 19 were evaluable for response. Patients were treated with escalating dosages of interferon alpha-2b: 3 x 10(6) IU, 6 x 10(6) IU and 12 x 10(6) IU. The escalation was performed every 8 weeks when no objective tumour regression was observed. Patients were also evaluated for biochemical response and symptomatic improvement. One objective tumour regression was observed. Of the 15 patients with elevated 5-hydroxyindole acetic acid (5-HIAA) excretion, 5 (33%) had a more than 50% decrease in 5-HIAA excretion. Relief of symptoms occurred in 11 patients (58%). This improvement was already apparent during the initial 8 weeks of treatment. Increasing the dose to 6 or 12 x 10(6) IU interferon alpha-2b did not result in further symptomatic improvement. In contrast toxicity was considerable with the higher dosages of interferon alpha-2b. It is concluded that low dose interferon alpha-2b (3 x 10(6) IU) three times weekly is as effective as higher dosages of interferon alpha-2b at ameliorating symptoms of the carcinoid syndrome.