A prospective study was carried out in 26 patients intubated and mechanically ventilated for severe exacerbation of asthma. The age of onset of asthma and the duration of disease showed wide variations, but most of the patients were women in their fifties with chronic asthma. However, 23% had the more benign intermittent course. Most patients had a prodromal period of several days with progressive deterioration. Eight patients developed sudden attacks, in three of them aspirin-intolerance was responsible, in two patients emotional disturbances could play some role as triggering factors, and in three other patients no obvious reason was found. Identification of factors responsible for the sudden attacks may be relevant to the management of these severe cases. The majority of patients were intubated immediately on arrival or during the first hours after admission. Deaths were related to complications produced by hypoxia, or hospitalization in an intensive care unit. Probably many of these patients would not have died if an appropriate steroid treatment had been given some days before.