The purpose of this study was to determine whether inactive asthmatic patients could perform high-intensity physical training equally well on land as in water, and to compare the effects of these training forms. Thirty-two adults with asthma, randomized into two groups, underwent a 10-week supervised rehabilitation program with emphasis on physical training. All patients, irrespective of training form, were able to exercise to maximal intensity (80-90% of estimated maximal heart rate). No asthmatic attacks occurred in connection with the training sessions. Respiratory variables remained almost unchanged in both groups. The asthma symptoms declined during the rehabilitation period, and the subjects needed less acute asthma care after the rehabilitation. The cardiovascular condition improved significantly and similarly in the two groups. Ten patients, 5 in each group, had exercise-induced asthma at the start of the rehabilitation. Only 3 patients, 2 from the water group and 1 from the land group, had exercise-induced asthma after 10 weeks. We conclude that indoor training, either on land or in water, is beneficial. The effects of these two training forms are almost equivalent.