In a prospective investigation lasting for one year, the medically staffed ambulance in Copenhagen registered patients under the age of 60 years with asthma who developed respiratory arrest outside hospital. The circumstances involved in the acute serious cases were illustrated as far as possible by review of the case records and interviews with the surviving patients. Five patients participated in the investigation. All of these were young with a median age of 24 years (19-28). Two patients had cardiac arrest, four patients survived after tracheal intubation at the site of the onset of illness and subsequent mechanical ventilation. One patient died in hospital on account of anoxic encephalopathy. The common denominators of all of the patients were that the condition had been recognized for many years, that before the attack involved they had been well and without asthmatic symptoms, the attack commenced hyper-acutely and the patients had not taken medicine regularly during a prolonged period. On the basis of these observations, the present authors recommend that young patients with asthma should receive optimal medication and that the necessity of taking the prescribed medicine despite possible good health should be emphasized.