The families of 32 children with cystic fibrosis (CF) were interviewed about both their tobacco consumption and their childrens physical activities. Hospital records informed about treatment frequency, lung function and clinical score. Cystic fibrosis families smoked far more than the Swedish average and the passive smokers among our patients seemed to fare less well in all parameters. The children of smoking mothers required significantly longer periods of intravenous antibiotic treatment (P greater than 0.05). Frequent physical exercise seemed to compensate for the potential harmful effects of passive smoking and children with high physical activity living in families who smoked needed significantly less frequent antibiotic treatment than the inactive children (P greater than 0.02). Although this series is small, the results indicate that a smoke-free environment may be important for CF patients. General information is insufficient and extensive psychological support to the families is probably necessary.