Two fatal and five non-fatal episodes of air embolism occurred in the course of 14,000 haemodialyses. Air entered the circuit through the infusion bottle, the heparin syringe or line, or the blood pump insert. Emergency treatment with posture, oxygen, dextran infusion, and dexamethasone was apparently successful in reversing some of the manifestations. Prevention depends partly on better design of equipment and partly on the incorporation in the haemodialysis circuit of devices which detect air and prevent it from reaching the patient. Such monitors are not in general use in the United Kingdom and those currently available are far from infallible. We describe an alternative system incorporating a floating valve in the bubble trap.