Pulmonary surfactant maintains patency of narrow conducting airways. An inflammation, with a leakage of plasma proteins into the airway lumen, causes surfactant to lose some of this ability. Will a lowering of temperature aggravate the deteriorating effect of an inflammation? Calf lung surfactant extract (CLSE) with proteins added was studied with a capillary surfactometer (CS) at temperatures of 25-42 degrees C. BALB/c mice were infected with respiratory syncytial virus (RSV). Six days later the lungs were lavaged and the surfactant in the lavage fluid was studied with the CS at temperatures of 25-42 degrees C. Lavage fluid from allergen challenged asthmatics was examined for its content of surfactant inhibitors at reduced temperatures. It was shown that CLSE with proteins gradually lost its ability to maintain patency as the temperature was lowered. Lavage fluid from the RSV infected mice showed a similar dysfunction at low temperatures. Lavage fluid from the airways of human asthmatics, when challenged with antigen but not with saline, contained agents inhibiting surface activity, particularly at reduced temperatures. Airway inflammation causes surfactant to lose its ability to maintain patency, particularly as the temperature is reduced. That might be a reason for the increased airway resistance observed in asthma patients hyperventilating in cold weather.