Cystic fibrosis sufferers can prolong their life expectancy by taking regular exercise.
Some 109 American sufferers aged between 7 and 35 were tested for aerobic fitness in the late 1970s and then tracked for eight years.
Those with the highest levels of fitness had an 83 per cent survival rate at the end of the study, compared with survival rates of 51 per cent and 28 per cent for those with middle and low fitness levels.
"After adjustment for other risk factors, patients with higher levels of aerobic fitness were more than three times as likely to survive as patients with lower levels of fitness," the researchers conclude.
They suggest that lack of aerobic fitness itself may simply be a marker for the severity of the condition, but add:
"Even if fitness does reflect more severe illness, it is still strongly correlated with survival and is thus a powerful marker, suggesting that exercise testing is a useful tool. If, on the other hand, better aerobic fitness itself imparts a better prognosis, this would be important to know, since it would encourage the use of exercise training as a therapeutic measure."