Disease transmission and barrier techniques during dental treatment have been areas of recent concern in dentistry. Ultrasonic scalers are known to produce aerosols, and these aerosols are often produced from areas of significant disease activity, including bleeding. This study was performed to determine if these aerosols contain blood from the gingival sulcus. Forty areas consisting of two contiguous periodontally involved teeth (probing depth of at least 5 mm on one site of each tooth) were scaled subgingivally with an ultrasonic scaler for 30 seconds. A high volume evacuator (HVE) tip was positioned 3 to 5 cm away from the operating site and utilized to capture the aerosols produced. The water remaining in and on the HVE tube was tested for occult blood by the guiac resin method. Gingival index, mean probing depth, presence of bleeding with scaling, and presence of visible blood in the HVE tip were recorded. All 40 test sites showed a positive result for blood in the captured aerosols despite the wide variation in the measured parameters. It may be concluded that subgingival scaling on periodontally involved teeth with ultrasonic scalers would be expected to produce aerosols containing blood. J Periodontol 1998;69:434–438.