Hypokalaemia is a common finding in hospitalized patients. In most cases the cause will be obvious. However, in a subgroup of patients the cause is occasionally uncertain and establishing the diagnosis may present difficulties and can become a challenge. In such cases, measurement of urinary indices e.g. urine potassium/creatinine concentrations along with blood acid/base parameters and assessment of urinary excretion potassium, chloride, calcium and creatinine have been employed in the differential diagnosis. This article presents a case report of an adult male with severe resistant hypokalaemia which an initial potassium level of one mmol/L and associated with limb weakness, metabolic alkalosis and hypercalciuria. His potassium level was refractory to oral and intravenous replacements. This patient showed a marked increase of serum potassium after administration of indomethacin. This patient had the full biochemical features of Bartter syndrome (BS), there was a severe hypokalaemia, with inappropriate urine K+ loss, with severe urinary chloride excretion associated with mild metabolic alkalosis and normal blood pressure. He had also hypocalcaemic hypercalciuria, His urine calcium/creatinine ratio was 0.26 which was indicative of hypercalciuria. His age and late symptom presentation would have suggested Gitelman disease but due to hypercalciuria, the clinical diagnosis of Bartter Syndrome was made. We describe here a case of Bartter Syndrome type 3 whose diagnosis was made in adult life. This patient is an unusual, possibly extreme case of BS. Being 30 years old at the time of diagnosis, he is by far the oldest patient so far described with BS. Calculating the urinary potassium/creatinine ratio, urinary chloride excretion and urinary calcium/creatinine ratio in conjunction with plasma acid/base values provided simple and reliable tests to distinguish Bartter syndrome from other differential diagnosis of hypokalaemia and to provide a proper management in the intensive care unit until further advanced investigations to be done.