In the last decade hyperbaric oxygen has been recognized as an important therapeutic tool in a variety of instances in which either destruction of anaerobic bacteria is urgent or an improvement in the oxygenation level is mandatory. We have used it successfully in a case of mediastinal anaerobic infection (gas gangrene) after medical and surgical measures had failed to eradicate the disease. The causative organism of gas gangrene, Clostridium perfringens (Welchii), is widely distributed. It may be cultured from the soil, house dust, human skin and faeces, etc. For this reason infection with Clostridium is practically inevitable whenever suitable conditions arise. As an anaerobic bacterium Clostridium Welchii multiplies readily in damaged tissues without contact with the air and devoid of a normal blood supply. This occurs especially in road accidents and war wounds in which broken bones and crushed muscles provide a suitable medium for the infection (Altmeier, 1965). Sometimes this infection can occur too after abdominal or gynaecological operations (Hitchcock, 1965). We present here a case of mediastinal gas gangrene which was caused by perforation of the oesophagus by a swallowed foreign body.