The purpose of this study was to describe both the level and relationship of physical activity and exercise in children with cystic fibrosis of varying degrees of severity. Thirty-six children (19 girls and 17 boys) between 6 and 19 years of age with cystic fibrosis underwent pulmonary function and graded treadmill testing. Physical activity was measured with a 7-item Likert-scaled Activity Questionnaire completed by parents (0.87, Alpha coefficient). Subject's severity of disease classifications based on pre-exercise pulmonary functions were as follows: 61% were in the normal category; 26% in mild; 8% in moderate; and 5% in severe. Activity scores ranged from 5 to 33 (x = 22. +/- 6.80). The subjects' levels of fitness were low as determined by their maximal oxygen uptake values. Correlations showed no significant relationships between the activity scores and oxygen uptake measures. Nurses can develop individualized exercise regimens based on clinical and psychosocial data as well as exercise testing for children with cystic fibrosis and their families.