[Insulinoma with hyperproinsulinemia: a two cases report].

We report two cases of insulinoma with increased serum proinsulin levels and with normal insulin levels.Clinically, the two patients show typical symptoms of insulinoma, but serum insulin levels are normal. Only high levels of serum proinsulin allow us to retain the diagnosis of insulinoma. The new insulin assays using monoclonal antibodies are very specific for insulin and though can not detect isolated high proinsulin levels. So proinsulin assay is useful when diagnosing insulinoma.