ABSTRACT Bruce Lee brought attention to martial arts in the Western world and popularized the quote ‘Be water, my friend’. Lee died at the age of 32 years in Hong Kong on 20 July 1973, under mysterious circumstances. The cause of death is unknown, although numerous hypotheses have been proposed, from assassination by gangsters to the more recent suggestion in 2018 that he died from heatstroke. The necropsy showed cerebral oedema. A prior episode was diagnosed as cerebral oedema 2 months earlier. We now propose, based on an analysis of publicly available information, that the cause of death was cerebral oedema due to hyponatraemia. In other words, we propose that the kidney’s inability to excrete excess water killed Bruce Lee. In this regard, Lee had multiple risk factors for hyponatraemia that may have included high chronic fluid intake, factors that acutely increase thirst (marijuana) and factors that decrease the ability of the kidneys to excrete water by either promoting secretion of antidiuretic hormone (ADH) or interfering with water excretion mechanisms in kidney tubules: prescription drugs (diuretics, non-steroidal anti-inflammatory drugs, opioids, antiepileptic drugs), alcohol, chronic low solute intake, a past history of acute kidney injury and exercise.