The relationship between passive exposure to cigarette smoking and objective measures of health was examined in 340 patients with cystic fibrosis attending a large hospital-based clinic. Patients who came from households with smokers did not differ from those living in smoke-free households in terms of nutritional status, clinical scores, spirometry, or colonization with Pseudomonas. The number of cigarettes smoked in the household was not significantly related to nutritional status, clinical score, spirometry, or hospitalization. Similar results were found when children 6 to 11 yr of age were analyzed separately, except that height percentile was negatively related to the number of cigarettes smoked in the household. The effects of household exposure to cigarette smoke were further evaluated by analyzing changes in nutritional status, clinical score, and spirometry over a 15-yr period among patients whose families never, always, stopped, or started smoking during this time. Height percentile increased slightly during this interval among those whose households never smoked, whereas no change occurred among patients whose households always smoked, and a decline was seen among patients whose households quit. These differences were statistically significant. Patients whose households never smoked had consistently higher pulmonary function measurements than did patients whose families always smoked, although the differences were not statistically significant. The rates of decline were similar in these two groups. Patients whose households stopped smoking had significantly lower pulmonary functions at the end of the study than did subjects whose households never smoked. When patients who were homozygous for the ΔF508 mutation were analyzed separately, results were similar. The longitudinal analysis suggests that a “healthy smoker effect,” whereby families of individuals least affected by respiratory disease are more likely to continue smoking, may have occurred.