Cystic fibrosis is characterized by chronic obstructive lung disease and malnutrition. Previous studies have shown that nutritional status and lung function are limiting factors for exercise capacity. A reduced exercise capacity may in turn diminish activity levels. We evaluated whether the total time spent somewhat active (e.g., walking) or active (e.g., biking), as reported by the Habitual Activity Estimation Scale, was related to lung function, as evaluated by forced expiratory volume in one second (%predicted FEV1), and nutritional status, measured as body mass percentile, in 36 children with cystic fibrosis, aged 6 to 16 years. The Habitual Activity Estimation Scale questionnaires were completed by the parents for children younger than 12 years of age and by both the parent and the child, independently, for those 12 years and older. Patients had a body mass percentile of 99 +/- 15.2% and % predicted FEV1 of 85.7 +/- 20, with no differences between boys (15/36) and girls (21/36). Boys spent 8.1 hours and girls spent 7.5 hours (P > 0.1) being at least somewhat active. These values are similar to those reported for healthy boys and girls. In patients with significant lung disease (%predicted FEV1, < or = 75; n = 11), activity level (the time spent somewhat active or active) was related to nutritional status (r = 0.675; P = 0.02) but not to lung function (r = 0.21; P > 0.1). Activity level reported by patients 12 years of age and older was on average 24.1% higher (P < 0.05) than that reported by their parents, but the two reportings were related (r = 0.758; P = 0.004). These results suggest that activity level may be restricted by nutritional status in those patients with significant air flow limitation. We suggest that improving the nutritional status of cystic fibrosis patients may prevent decreases in activity levels and quality of life of these affected children.