A 39-year-old woman presented with severe, uncontrolled and irreversible hypomagnesaemia, following cisplatin treatment in her childhood. Because high-dose oral magnesium supplementation therapy was insufficient and not tolerated, continuous subcutaneous magnesium supplementation was successfully instituted and continued in the outpatient setting. This case demonstrates that continuous subcutaneous magnesium supplementation is effective in maintaining magnesium levels within the normal range, is well tolerated and may provide a long-term solution for chronic hypomagnesaemia due to intractable renal losses.