Hypoglycaemia as a result of insulin secreting tumours can be treated effectively by surgery. However pre-operative localization is desirable as the majority of these tumours are small and in 20% of the cases may be multiple. We describe a 1 cm insulinoma being localized pre-operatively by selective arterial stimulation with calcium and venous sampling for insulin. This technique is sensitive and relatively non-invasive. We believe that it is potentially very valuable in pre-operative localization of occult insulinomas.