The onset of fatal and near-fatal asthma attacks can be rapid. The objective of this study was to determine the frequency and clinical characteristics of rapid-onset asthma (ROA) in patients suffering fatal and near-fatal crises. Two-hundred and twenty patients with fatal or near-fatal attacks were enrolled in a multicentre, prospective study. ROA was defined as a crisis developing in < or =2 h. Data on patient and clinical characteristics were collected, and spirometric and allergy studies were performed when the patients were in a stable condition. Forty-five attacks (20%) were ROA and 175 (80%) were slow-onset asthma (SOA). The triggers for SOA and ROA attacks were different, with the ROA group having a significantly lower rate of suspected respiratory infection (7% versus 38%), higher rates of fume/irritant inhalation (9% versus 1%) and a higher intake of nonsteroidal antiinflammatory drugs (14% versus 3%). The ROA group exhibited significantly higher rates of impaired consciousness (63% versus 44%), absence of lung sounds upon admission (68% versus 42%), fewer hours of mechanical ventilation (13 h versus 28 h) and fewer days of hospitalization (8 days versus 9.5 days) than the SOA group. The 20% frequency of rapid-onset fatal and near-fatal attacks in this study suggests that rapidly developing attacks may not be rare. These findings also support a distinct clinical profile for rapid-onset asthma marked by differences in triggers, severity of exacerbation and clinical course.