To determine the effects of acute viral respiratory infections (ARI) in cystic fibrosis (CF) we studied all episodes of ARI in ten patients over a 2 year period. A daily diary card was kept on which they recorded all illnesses, including specific upper and lower respiratory symptoms, antibiotic use, and daily peak flow measurements. Recording of concurrent similar illnesses in other household members aided in confirming the onset of ARI, as did a home viral culturing technique and serial serum antibody levels against specific viral types. A total of 80% of the 35 recorded onsets of ARI (1.75/yr/subject) were confirmed by one or more of these methods. Five subjects with moderate to severe pulmonary function scores at the outset of the study had significantly greater decreases in PEFR during ARI episodes (42.4%) than those with milder disease (15.4%), and it took them significantly longer to recover to their baseline measurements (22 days vs. 15 days respectively). The majority of hospital admissions for pulmonary exacerbations were preceded by viral infections (7/8).