A study of the exposure of operating room personnel to blood containing aerosols was conducted. Breathing zone aerosol samples were collected from the primary and first assistant surgeon during 18 orthopedic, urologic, cardiothoracic, vascular, or obstetric procedures using personal cascade impactors having effective particle aerodynamic diameters of 14, 3.5, and 0.52 micrometers. These cutoff points corresponded to stages 2, 5, and 8 of the cascade impactor, respectively. The degree of blood contamination of the collected aerosols was assessed by determining their hemoglobin (Hb) content using hemastix strips. Exposure concentrations for primary surgeons ranged from nondetectable to 7.4 micrograms Hb per cubic meter (microg Hb/m3), mean 1.4microg Hb/m3. Exposure concentrations for surgeon assistants varied from 0.3 to 4.8microg Hb/m3, mean 1.8microg Hb/m3. Approximately 90% of all samples collected by stage 2 of the impactor, 66% of those collected by stage 5, and 38% of those collected by stage 8 were positive for Hb. Stage 5 included particle sizes capable of depositing in the tracheobronchial and gas exchange regions of the respiratory tract. Stage 8 also collected particle sizes that could deposit in the gas exchange region. The authors conclude that primary and assistant surgeons are exposed to inhalable blood containing aerosols while performing surgical procedures. Until the potential of these aerosols to transmit HIV or other bloodborne pathogens is determined, the authors recommend that operating room personnel wear respirators instead of surgical masks.