In this paper we develop a framework to categorize the concepts used to estimate indirect costs in economic evaluation. We apply this framework in a literature review of economic evaluation studies. We searched all English language literature from 1994-96. Following the application of a search algorithm, which yielded 25 articles, we abstracted information from these articles to determine the methods used to identify the relative contribution of indirect costs of the cost-effectiveness ratio, the time horizon selected by the authors, the identification of those activities that were foregone, the time given up by these activities, and the valuation placed on this time. These methods were then assessed. Indirect costs, as they have been measured, significantly influence efficiency ratios. A wide variation exists among studies in how they incorporate each of the components of indirect costs. All of the four components that were identified will affect the measurement of indirect cost. Future guidelines in this area should address the entire measurement process.