In view of the lack of objective information on work performance in asthma, a progressive incremental exercise test was carried out in 44 subjects with mild to moderate asthma and 64 normal, healthy subjects matched for habitual activity, to compare cardiorespiratory fitness and to determine the relative contribution of airflow obstruction to exercise limitation. The two groups achieved similar maximum heart rates (mean (SD) 176(12) and 175(10) beats/min). After allowance for confounding factors the asthmatic subjects had a lower maximum oxygen consumption (VO2 max) (by 199 ml min-1) than control subjects. Having asthma also accounted for a significant reduction in anaerobic threshold (125 ml min-1) and oxygen pulse (0.805 ml/beat). There was no correlation of FEV1 with VO2 max, anaerobic threshold, or oxygen pulse either before or after bronchodilator. The dyspnoea index (VE/MVV%) was increased in the asthmatic subjects at peak exercise, but was less than 60% in all subjects at a workload that produced 75% of the predicted maximum heart rate. Thus the asthmatic subjects had a maximum heart rate similar to that of normal subjects but the low VO2 max, anaerobic threshold, and oxygen pulse suggest suboptimal fitness, which was not directly due to airflow obstruction. All had sufficient ventilatory reserve to allow toleration of training at a work intensity adequate to permit improvements in cardiovascular fitness.