OBJECTIVE: Meta-iodo-benzyl-guanidine labelled with 131-iodine (131I-mIBG) has been used extensively for imaging tumours originating from the neural crest but experience with its therapeutic use is limited, particularly for non-catecholamine secreting tumours. In order to assess the therapeutic response and potential adverse effects of the therapeutic administration of 131I-mIBG, we have reviewed all patients who had received this form of treatment in our department. DESIGN: Retrospective analysis of the case notes of patients with neuroendocrine tumours who received treatment with 131I-mIBG and were followed-up according to a defined protocol in a given time frame. PATIENTS: Thirty-seven patients (18 with metastatic carcinoid tumours, 8 metastatic phaeochromo-cytoma, 7 metastatic paraganglioma and 4 metastatic medullary carcinoma of the thyroid) treated with 131I-mIBG over a 15-year period were included in this analysis. MEASUREMENTS: The symptomatic, hormonal and tumoural responses before and after 131I-mIBG therapy over a median follow-up duration of 32 months (range 5-180 months) were recorded. Of the 37 patients (22 males; median age 51 years, range 18- 81 years), 15 were treated with 131I-mIBG alone whereas the other 22 received additional therapy. RESULTS: A total of 116 therapeutic 131I-mIBG doses were administered [mean cumulative dose 592 mCi (21.9 GBq); range 200-1592 mCi (7.4-58.9 GBq)]. None of the patients showed a complete tumour response. However, 82% of patients treated with 131I-mIBG alone and 84% who received additional therapy showed stable disease over the period of follow-up. Overall survival during the period of the study was 71%. The overall 5-year survival rate was 85% (95% confidence interval, 72-99%) for all patients and 78% (95% confidence interval, 55- 100%) for the carcinoid group alone, according to Kaplan-Meier analysis. Symptomatic control was achieved in all the patients treated with 131I-mIBG alone, and in 72% of those receiving additional therapy. Hormonal control was noted in 50% and 57% of patients, respectively. 131I-mIBG therapy was safe and well tolerated. Serious side-effects necessitating the termination of 131I-mIBG therapy were seen in only 2 of our patients. CONCLUSIONS: 131I-mIBG therapy produces symptomatic and hormonal improvement and moderate tumour regression/stabilization in patients with meta-static neuroendocrine tumours with minimal adverse effects. It may be a valuable alternative or additional therapeutic option to the currently available conventional treatment modalities.