A survey of UK patients receiving the drug diazoxide, revealed 40 patients with insulinoma on this treatment. Mean age (+/- SD) was 67 +/- 18 years, and 74% were female. Duration of treatment was 7 +/- 6 years (range 1-22). Most (55%) patients were treated with diazoxide because of tumour non-localisation (including failed previous surgery). Metastatic disease (20%) and poor surgical risk (10%) were other indications. Side-effects (notably fluid retention and hirsutism) were common (47%) but not troublesome. Treatment was highly effective--59% were symptom free and 38% had only occasional symptoms. Only one patient had frequent hypoglycaemia despite treatment. We conclude that diazoxide is effective in the management of insulinoma. Side-effects are common but not problematic. Treatment should be considered for all patients not cured by surgery, or unsuitable for surgical treatment.