Due to infection risks, high demands are set for the air quality in operating theatres. This is normally realised through the use of a downflow plenum. In this article the application of a fluid flow simulation program (CFD), as a design tool for the evaluation of a new ventilation design for operating theatres, is discussed. In this case an alternative downflow plenum has been investigated, that differs from the currently common designs. The use of CFD allowed for the examination of important design criteria and the resulting adjustments to the design. By means of this study a better understanding of the effect of relatively small adjustments could be gained. Examples are, the use of a partition and the free space to instrument tables. However, the positioning of the recirculation exhaust especially, had a large influence on the flow in the room.