Although exercise may be beneficial in cystic fibrosis (CF), patients' low tolerance to climatic heat stress means that physical exertion can increase morbidity and mortality. We postulated that the high salt content of CF patients' sweat and the consequent absence of body-fluid hyperosmolality during a long episode of sweating might deprive such patients of a thirst stimulus. Eight children with CF (four boys, four girls; aged 9.5-14.1 years) and eight controls, matched for age and sex, attended two randomly ordered sessions of exercise (cycling) in a chamber at 31-33 degrees C, relative humidity 43-47%. 20 min bouts of exercise (at 45% of predetermined maximum oxygen uptake) were interspersed with 25 min rest periods. At one session, chilled water was given every 15-20 min to replace fluid lost; at the other, drinking was guided by the child's thirst. At the thirst-guided session, CF patients drank much less than the controls did (0.80% vs 1.73% initial body weight) and lost twice as much fluid (1.57% vs 0.78% initial body weight). The recovery of heart rate after exercise was slower in CF patients, but there were no other signs of heat strain. The groups did not differ in any variable during the forced drinking session. We conclude that children with CF underestimate their fluid needs and undergo excessive dehydration during extended exposure to hot conditions.