1. The intensity of breathlessness in normal subjects during exercise was measured on seven occasions over a 40-week study period to assess the long-term repeatability of Borg scale estimates of breathlessness. 2. In all subjects there was a significant correlation (P = 0.0001) between breathlessness and minute ventilation. Minute ventilation measured at each work rate did not differ between the seven exercise tests (P >0.05). 3. There was no significant difference between the mean Borg scores (measured with respect to a given level of ventilation) in 5 of the 7 test weeks (P >0.05). The slope of the relationship Borg score/minute ventilation did not differ between the seven exercise tests (P >0.05). 4. Breathlessness estimation was highly reproducible both after 1 week and after 40 weeks of the study (both P >0.05). 5. The duration without testing between consecutive tests did not affect reproducibility: the mean Borg score was as equally reproducible after an interval of 22 weeks without testing as after an interval of 1 week (P >0.05). 6. The Borg scale provides a reliable technique for studying the sensation of breathlessness over extended time periods.