We report a case of postoperative hyponatraemia following routine knee surgery, followed by a subsequent, less severe, episode after identical surgery on the contralateral knee. On each occasion the patient had been given the weak opioid tramadol for postoperative pain relief. Through its effects on serotonergic neurotransmission in the central nervous system, we hypothesise that tramadol may have been directly involved in this patient's biochemical disorder.