Background: Insulinomas are β-cell tumours characterised by uncontrolled insulin secretion even in the presence of hypoglycaemia. However, the mechanisms allowing such excessive insulin secretion are not known. Insulin secretion can occur only when the β-cell insulin stores have been replenished by insulin biosynthesis, which is mainly controlled by translation. Such specific translational regulation often involves the 5′ untranslated region. We have identified an insulin splice variant in isolated human pancreatic islets of non-diabetic donors that retains 26 bp of intron 1 and thereby changes the 5′ untranslated region, but leaves the coding region unchanged. This splice variant has increased translation efficiency in vitro and in vivo compared with native insulin mRNA. However, splice variant expression is less than 1% of native insulin mRNA in normal islets. Methods: To test whether this splice variant is involved in insulin production by human insulinomas, we extracted RNA from nine laser-captured surgical insulinoma samples and from isolated islets of nine donors who did not have diabetes. We then determined the ratio of splice variant to native insulin mRNA by quantitative real-time RT-PCR. Findings: The mean ratio of the splice variant to native insulin mRNA was increased more than 50-fold in insulinomas compared with normal islets, and this difference was present in all nine human insulinomas. Overexpression of the splice variant therefore seems to be a general characteristic of insulinomas and is estimated to contribute about 90% to insulin synthesis by these tumours. Interpretation: Overexpression of the insulin splice variant with increased translation efficiency in insulinomas might explain how these tumours maintain high levels of insulin synthesis and secretion leading to hyperinsulinaemia - the hallmark of this disease.