The speed of onset of attacks of acute severe asthma was prospectively studied in a total of 261 consecutive episodes. 82% of these episodes involved patients with 'extrinsic' asthma and 28% involved children. The speed of onset of an attack was rapid (defined as less than 24 hours) in 46% of episodes and was less than one hour in 13%. Rapid-onset attacks occurred more frequently in younger patients and were no more or less severe than attacks of slower evolution. Comparison of different measures of severity in our patients showed statistically significant but low correlations. Objective measures seemed no better than simple clinical assessment with the Jones index. We discuss the significance of these correlations and the place of emergency admission services for attacks of acute severe asthma of rapid speed of onset.