Workflow Management (WfM) systems have acquired a respectable place in the market of enterprise information systems. Although it is clear that implementation of a WfM system may shorten process execution and increase efficiency, little is known about the extent of these effects on business process performance. In this paper, we report on a running longitudinal multi-case study into the quantitative effects of WfM systems on logistic parameters such as lead time and service time. We conclude that in most cases significant decreases of lead time and service time will take place for the cases under consideration. In the presentation of our research outline, we show how we use process simulation for the validation of our measurements, the prediction of performance improvement, and the comparison of the pre-and postimplementation situation. As a side effect of this study, we present some interesting characteristics of actual business processes and the way WfM systems are implemented in practice.