Sixty-five children with severe asthma were tested on a bicycle ergometer to determine their cardiopulmonary endurance and to determine whether they could achieve normal levels of endurance after a physical rehabilitation program. Workloads on entry into the study ranged from 12% to 222% (median 70%) of predicted. Abnormalities in cardiopulmonary endurance did not correlate with historical features commonly used to define severe asthma or degree of airways obstruction. Forty (58%) patients required further bicycle ergometry training two to five times per week: 32 had scores less than 2 SD below the mean, and eight had scores between 1 and 2 SD below the mean. No adverse side effects were encountered during either testing or training. Of the children with scores less than 2 SD below the mean, 84% achieved normal workloads within 2 to 17 weeks of hospitalization. Rates of improvement were similar in the 27 children who subsequently passed and the five children who did not pass, suggesting that duration of program participation was the most important variable in determining success. Thus severely asthmatic children with abnormal fitness levels can achieve normal cardiopulmonary fitness after training on the bicycle ergometer.