Carbon monoxide (CO) poisoning is the commonest single cause of fatal poisoning in the U.K. (Broome & Pearson, 1988). The clinical features are numerous and include headache, fatigue, dizziness, confusion, memory loss, paraesthesia, chest pain, abdominal pain, nausea, and diarrhoea as well as coma, convulsions and death. Without adequate treatment many patients develop neuropsychiatric sequelae including headaches, irritability, memory loss, confusion and personality changes. The diagnosis of CO poisoning is often suggested only by circumstances surrounding the victim, and remains a challenge to the A&E department. Hyperbaric oxygen therapy (HBO) is internationally accepted as the most powerful form of treatment in severe cases (Drug & Therapeutics Bulletin, 1988; Lowe-Ponsford & Henry, 1989). However, in the U.K. treatment with HBO is often not considered due to lack of hyperbaric facilities (Meredith & Vale, 1988; Anand et al., 1988), and due to inadequate awareness on the part of hospital staff. We report a case of a patient deeply unconscious as a result of CO poisoning, in which serial treatments with HBO over a period of 14 days, produced dramatic results.