One case of pseudo-hyperkalaemia is reported in a women who suffered of labile hypertension and was previously treated by fluorohydrocortisone because of hyperkalaemia. Clinical examination and ECG were normal. An incubation test performed at different temperatures confirmed the diagnosis. There was no haematological abnormality. Familial screening prompted us to discover 2 affected subjects among four studied, according to autosomal dominant inheritance. This abnormality was due to increased passive membrane permeability to potassium at low temperature out of any hematological syndrome. The diagnostic is important to avoid inappropriate medications.