Many resources are expended to ensure a sterile operating room environment. Efforts are made to prevent exposure of patients to personnel and to achieve positive room pressure to keep out airborne contaminants. Foot traffic into and out of the operating room during surgery can undermine these efforts. The authors investigated the number and duration of operating room door openings during hip and knee arthroplasty procedures and the effect of the door openings on room pressure. They tested the hypothesis that door openings defeat positive pressure, permitting air flow into the room. Room pressure and door status were monitored electronically during 191 hip and knee arthroplasty procedures. Operating room staff were unaware that data were being collected. The authors evaluated the data with regression analysis to determine whether the number and duration of door openings had an effect on room pressure. Significance was set at P<.05. Doors were open, on average, 9.5 minutes per case. In 77 of 191 cases, positive pressure was defeated, allowing air flow to reverse into the operating room. Total time with the door open significantly affected the minimum pressure recorded in the room (P<.02), but did not significantly affect average room pressure (P=.7). This finding suggested that the loss of positive pressure was a transient event from which the room recovered. The number and duration of door openings showed a significant association with length of surgery. Door openings threaten positive pressure, potentially jeopardizing operating room sterility. The causes of excessive operating room traffic must be evaluated to identify ways to reduce this traffic and the associated risks.