Respiratory deterioration accounts for the morbidity and mortality observed in patients with cystic fibrosis. The role of Legionella in this deterioration was determined in a 2-year prospective study of 49 patients with cystic fibrosis and 19 sibling controls. Sera were obtained from participants on enrollment and at quarterly intervals. Legionella antibodies were measured in parallel using an indirect fluorescent assay. No seroconversions were observed. Eleven of 49 patients with cystic fibrosis (22%) were seropositive compared to none of 19 siblings (P less than 0.05). Six of the 11 patients demonstrated high titers (greater than or equal to 1:512) that persisted throughout the study. Absorption with pools of various Pseudomonas species reduced the antibody titers such that only 3 remained positive after absorption. Legionella was not found to be an important cause of clinical deterioration during this study. The results of the absorption studies suggest that high titers to Legionella in this population are due to cross-reacting antibodies.