We are interested in the design of operating theatres and also the effect of changes in clinical practice on airborne bacterial contamination. To study airflow in the theatre we used a Windmaster 3D ultrasound based anemometer. The device was used in an empty operating theatre, to map the airflow in 3D and to study the effect of movements by theatre personnel around the periphery of the clean zone. We also measured the effect of opening and closing the theatre doors. During surgery we used the anemometer to measure the effect of different positions of the surgical lights and the surgical drape, which forms the anaesthetic screen. We found that the ultraclean zone of the operating theatre performed to specification, but there were relatively stagnant areas elsewhere. Airflow was altered by personnel walking rapidly past the corners of the ultra clean zone. We were able to identify and optimum configuration of the theatre lights and drapes, to produce the most favourable airflow pattern.