Spirometry is a widely used pulmonary function test that allows measurement of forced vital capacity and time-related measures of dynamic pulmonary function. This study was designed to identify the risk of cross-transmission associated with two commonly used dryrolling seal spirometers. Using a prospective study design, we examined whether microbial contamination of spirometers occurred following use by patients with a heavily colonized or infected respiratory tract. Prior to spirometry evaluation, a patient's sputum culture and equipment samples (i.e., mouthpiece, proximal tubing, piston surface) were obtained. After patient evaluation, a sterile 2 L ventilation bag and sterile tubing were used to simulate the risk of infection of subsequent patients. Simulation 1 was performed immediately after patient testing and Simulation 2, representing a second patient was conducted approximately 18 hours later. This study was conducted at the University of North Carolina Hospitals, a large university teaching facility. Fourteen patients with underlying pulmonary disease were studied. Our study revealed that the mouthpieces became contaminated with the patients' oral flora and with the associated respiratory pathogen. Fourteen percent of the associated tubing after patient testing contained the respiratory pathogen. All other equipment samples (e.g., interior surfaces of the machine, Simulation 1, Simulation 2 samples) were negative for the respiratory pathogen. These data suggest that mouthpieces and spirometry tubing may become contaminated with microorganisms and should not be shared between patients. Since there is little or no bacterial contamination of the surfaces inside the spirometers and cross transmission is unlikely, it is unnecessary to routinely clean the interior surfaces of the spirometers.