A 28-year-old woman with a depressive episode developed hyponatraemia (126 mmol/l), in the absence of hypovolaemia, oedema, or diuretic use, 2 weeks after starting treatment with the antidepressant venlafaxine. Full blood count, potassium, urea, creatinine, thyroid function test, liver function test, amylase, serum cortisol, and glucose were all normal. A repeat blood test 3 days later revealed a further reduction in the sodium concentration to 123 mmol/l. The patient did not have any signs or symptoms of physical disorder secondary to hyponatraemia on this occasion. Her serum sodium reached normal values 2 weeks after stopping treatment with venlafaxine. Hyponatraemia secondary to venlafaxine has been reported in the older population but not in young people. We therefore believe this is an important finding.