Carbon monoxide poisoning is the commonest cause of death by poisoning in the UK and chronic exposure is thought to be a frequently missed diagnosis. Early recognition of carbon monoxide poisoning is vital to institute prompt treatment and to prevent exposure to others. An incident of mass exposure to carbon monoxide is presented where euphoria, lasting several hours, was the only symptom reported in approximately one quarter of the casualties. This has not been reported previously and we believe that mild carbon monoxide intoxication should be included in the list of differential diagnoses of inappropriate euphoria.