Hypoglycaemia is rare in apparently well patients without drug-treated diabetes mellitus and warrants evaluation and management when Whipple's triad is present. Even in the absence of Whipple's triad, when repeatedly low values of plasma glucose are documented, the presence of endogenous hyperinsulinism should be investigated. The authors describe a case of endogenous hypoglycaemic hyperinsulinism, its diagnosis and treatment and the challenges of determining its aetiology. CONCLUSIONS A high degree of suspicion is necessary when a hospitalized, apparently well, patient presents with episodes of hypoglycaemia with no apparent cause.Documented repeated low glucose values (<40-55 mg/dl) warrant investigation even when Whipple's triad is absent.The diagnosis, approach and treatment of endogenous hypoglycaemic hyperinsulinism is challenging and requires persistence, particularly if examinations fail to show the responsible lesion.