We describe an unusual case of hypoglycemic syndrome in a 69-year old woman with a proinsulin-only secreting pancreatic endocrine adenoma. The clinical history was highly suggestive of an organic hypoglycemia, with normal or relatively low insulin concentrations and elevated proinsulin levels. Magnetic resonance and computed tomography of the abdomen showed a 1 cm pancreatic nodule and multiple accessory spleens. The diagnosis was confirmed by selective angiography, showing location and vascularization of the nodule, despite no response to intra-arterial calcium. After resection, the hypoglycemic syndrome resolved. The surgical specimen was comprised of a neuroendocrine adenomatous tissue with high proinsulin immunoreactivity. Study of this unusual case of proinsulinoma underlines (i) the need to assay proinsulin in patients with hypoglycemia and normal immunoreactive insulin, (ii) the differential diagnosis in the presence of accessory spleens, (iii) the unresponsiveness to intra-arterial calcium stimulation, and (iv) the extensive evaluation needed to reach a final diagnosis.