Intraoral use of ultrasonic scalers may generate aerosols that contain infectious microorganisms and therefore pose a hazard to the health of the dental professional. The purpose of this in vivo study was to determine if an aerosol reduction device for an ultrasonic scaler would be effective in reducing the amount of contaminated aerosols produced during ultrasonic instrumentation. Twelve adult subjects participated in the study. A split-mouth design was utilized, and two treatments (in separate rooms) were performed on each subject: 1) ultrasonic scaling for 5 minutes with the aerosol reduction device; and 2) ultrasonic scaling for 5 minutes without the aerosol reduction device. The right or left side of the subject's mouth was randomly assigned to one of the two treatment groups. After instrumentation, the subject and operator remained motionless for 25 minutes during collection of aerosol particles. Air samples were collected on blood agar plates 6 inches from the subject's mouth. Replicate organism detection and counting plates were used to sample microorganisms present on the investigator's face shield. All plates were incubated for 3 days at 37 degrees C. The results, using the paired t-test (P < 0.001), indicate that the ultrasonic scaler without the aerosol reduction device had a significantly greater quantity of mean colony forming units (CFUs) 6 inches from the subject's mouth (45.1 +/- 28.9) than the ultrasonic scaler with the aerosol reduction device (2.6 +/- 3.6). No significant difference was evident in the number of CFUs found on the investigator's face shield. These data suggest that an aerosol reduction device is effective in reducing the number of microorganisms generated during ultrasonic scaling, therefore decreasing the risk of disease transmission.