In this paper we examine whether the current method of valuing time within economic appraisals is appropriate. The cost to society of time taken to use health care may differ from the private opportunity cost of such time. However, demand for health care may be affected by the private opportunity time cost and not by the value of time to society. In this paper we show that the private opportunity cost of time is a better predictor of demand for a screening service than the societal approach. It is important, therefore, to use the private opportunity cost of time when assessing the demand for, and impact of, a health care intervention.