We present a case report of chemotherapy induced renal salt wasting syndrome (RSWS) that was initially diagnosed and managed as syndrome of inappropriate secretion of antidiuretic hormone (SIADH), based on osmolality values as well as hydration status. The patient was receiving chemotherapy for metastatic testicular cancer. Progressive deterioration of electrolyte balance prompted the diagnosis of RSWS. This was confirmed by a high urinary sodium concentration, a simple but important investigation which is rarely requested in the initial investigation of hyponatraemia. Urine sodium concentration is high in RSWS but normal in SIADH. With chemotherapy playing such an important role in cancer management, the correct diagnosis of hyponatraemia in an oncology patient is vital in order to allow appropriate management. Although the distinctions between SIADH and RSWS can be very subtle, the management of these two distinct clinical situations is very different—namely, fluid restriction versus salt replacement.