Iodine-131 metaiodobenzylguanidine (131I-MIBG) has been used with success for the palliation of symptomatic, metastatic carcinoid tumours. However, only 70% of cases are MIBG-avid and tumour uptake is not always sufficient for therapy. At The Netherlands Cancer Institute 34 carcinoid patients with no or insufficient uptake were treated with escalating doses of unlabelled ('cold') MIBG. No objective remissions were recorded, but a palliative effect (i.e. subjective disappearance of symptoms and/or reduction of medication by more than 50%) was observed in 60% of cases (mean duration 4.5 months). In 24 of the patients undergoing therapy with 'cold' MIBG, total body scintigraphy using 37 MBq 131I-MIBG was performed before and after infusion of 'cold' MIBG. The biodistribution of 131I-MIBG and its tumour to non-tumour ratios were compared. After 'cold' MIBG the 131I-MIBG uptake in the salivary glands was suppressed in all patients, myocardial uptake in 21, and uptake in normal liver tissue in 14. Pulmonary uptake was increased in 13 patients. More importantly, the tumour to non-tumour (T/NT) ratios improved in 17 of the 24 cases (by 7.8-111.4% at 24 h). Of the initial six patients demonstrating a significant increase in the T/NT ratio, five have subsequently received combined treatment of 7.4 GBq 131I-MIBG following the administration of 'cold' MIBG (both by 4 h intravenous infusion), resulting in a good palliative response in four of them. These patients had previously been excluded from therapy with 131I-MIBG only. It is concluded that the administration of unlabelled MIBG may not only provide palliation to patients with carcinoid tumours, but may also alter the biodistribution of MIBG, enabling 131I-MIBG therapy to be used in cases not qualifying for this treatment due to insufficient tumour uptake.