The implications for portfolio behavior and asset prices of transaction costs are central to the analysis of numerous issues in economics. For example, questions involving the demand for the financial contracts issued by financial intermediaries are intimately tied to the existence of transaction costs. Thus the analysis of questions involving the nature of the demand for mutual fund shares, insurance contracts, mortgage loans, etc., and the form those contracts take require the explicit inclusion of transaction costs.

This paper reexamines the anomalous evidence concerning the efficiency of the listed options exchanges. We focus on the structure of trading costs in that market, and note several costs which generally have been ignored, the largest of which is the bid-ask spread. When we adjust the published trading rules for our estimates of these trading costs, the reported abnormal returns are eliminated.