Twenty-four hospitalized patients with cystic fibrosis were enrolled into a 2-d, double-blind, placebo-controlled, randomized crossover trial comparing albuterol inhalation aerosol with a saline placebo. Aerosols were administered with the first three of four chest physiotherapy sessions given 4 h apart. Spirometry was measured before and 45 min after 7:00 A.M. and 3:00 P.M. therapy and before therapy at 7:00 P.M. and 7:00 A.M. the next morning. The mean percent change in FVC, FEV1, and FEF25-75% at 7:00 A.M. was 10.7, 14.8, and 19.6% with albuterol versus 2.4, 1.0, and -0.8% with placebo (p = 0.0012, < 0.0001, and = 0.003, respectively). A greater than 8% change in FEV1 separated changes with albuterol versus placebo with 96% specificity and occurred in 75% of all patients with albuterol; 71% at 7:00 A.M. versus 24% at 3:00 P.M. The reduction in response at 3:00 P.M. (p < 0.01) was presumably due to prolonged effects of morning therapy ( > 4 h). Individual changes in spirometry were significantly more positive and homogeneous with albuterol versus placebo at both 7:00 A.M. and 3:00 P.M. The mean percent change for the FVC, FEV1, and FEF25-75 across the day (7:00 A.M. pretherapy to 7:00 P.M. pretherapy) was 8.1, 10.1, and 9.7% with albuterol versus 3.9, 3.5 and 2.6% with placebo (p = 0.029, 0.036, and 0.232, respectively). The more positive and homogeneous changes in spirometry with albuterol, along with greater changes in these measures across the day when compared with placebo, suggest that albuterol improves pulmonary function in a majority of hospitalized patients with cystic fibrosis.