The influence of regular exercise on breathlessness and its relationship to ventilation has not been studied previously. We have examined the effects of a training programme on eight patients (5 males, 3 females, mean age 20 years, range 17-27 years) with cystic fibrosis. Eleven minutes of exercise was undertaken each day for 2 months according to the Royal Canadian Air Force protocol. Daily diary cards were kept and the programme was supervised. Pulmonary function and responses to maximal exercise on bicycle ergometer were determined before and after completion of the training schedule. Breathlessness was assessed using visual analogue scales (VAS) and related to ventilation during submaximal exercise on two occasions prior to training, and at the end. Apart from a reduced RV (pre 1.90 +/- 1.11, post 1.20 +/- 0.28 P less than 0.05), all other indices of pulmonary function and exercise performance were unchanged. Before training the relationship of breathlessness to ventilation was highly reproducible for each patient. After training there was a statistically significant reduction in breathlessness but ventilation was unchanged. At a mean standardized ventilation of 31.6 litres/min the VAS scales were 26.7% pre- and 12.9% post-training (P less than 0.01). Breathlessness can be favourably influenced by exercise training independent of ventilation with a consequent improvement in submaximal exercise tolerance in patients with cystic fibrosis.