The clinical course and results of drug treatment for manifestations of the carcinoid syndrome are reviewed in 63 patients. The five-year actuarial survival in this group of patients was 48 per cent. The only markers of a poor prognosis that could be identified at diagnosis were marked weight loss and a high (over 1000 mumol/day) 5-hydroxyindole acetic acid excretion. The relative effectiveness of well-established drugs that either block 5-hydroxytryptamine synthesis or block 5-hydroxytryptamine receptors is reported, with respect to the different manifestations of the syndrome, and compared with a small group of patients treated with long-acting somatostatin analogue. In over one-third of patients, primary tumours were not detected on initial investigation, and in none of these did symptoms referrable to the primary site become apparent later.