A 35-year-old woman with known familial hypokalaemic periodic paralysis received general anaesthesia for reduction of bilateral breast hyperplasia. Uncomplicated general anaesthesia was pet-formed using a propofol target-controlled infusion, remifentanil infusion and bolus doses of mivacurium with neuromuscular function monitoring. Plasma potassium concentrations were controlled intermittently in the peri-operative period and supplemented to achieve normokalaemia. Despite continuous substitution, an episode of low plasma potassium concentration occurred during the recovery period; this was without any clinical signs of muscle paralysis or respiratory distress.