OBJECTIVES To show the importance of patients' time as a cost of health and medical care and to explain how to include it in costing studies without greatly increasing the work required for such studies. BACKGROUND Despite the decade-old recommendation of the Panel on Cost-Effectiveness in Health and Medicine, patients' time is rarely included in costing or cost-effectiveness analyses (CEAs). Studies of cancer care, smoking cessation, and diabetes self-management show that it can be a large part of an intervention's costs, sometimes larger than direct medical costs, and can potentially affect patients' willingness to undertake the intervention. MEASURING AND VALUING TIME: Good costing practice follows 2 principles: measure all important uses of a resource; and value it appropriately and in a way that is consistent with the valuation of other resources. Counts of formal medical services, already measured in most studies, can serve as the starting point for valuing patients' time, and would be a major step toward recognizing time costs, even when analysts cannot measure other uses of time. The concept of opportunity cost, often approximated by a market price, is the basis for valuing all resources. The reasons why the wage is a reasonable proxy for the value patients place on their own time are explained. Wage data are well measured and readily available. CONCLUSIONS Ignoring patients' time underestimates disease burden and biases cost-effectiveness results toward interventions that use more time. The tools and data to include patients' time are available and will improve if they are routinely used.