A leak was observed emanating from the bottom of an Affinity Fusion oxygenator through two of the vents of the gas exhaust. The leak was observed during cardiopulmonary bypass a few minutes after cross clamp was applied for AVR surgery. Closer inspection revealed foamed blood widespread across the bottom of the oxygenator which was obscured the source of the leak. The leak was estimated to be approx 2-3mls/min. No change in arterial line pressure, blood gas values, or aptient haemodynamics was apparent before or after detection of the leak, or throughout the case that could be attributed to the leak. There was also no evidence of additional leaks, air entrainment or haemolysis.