This paper describes benefits and costs of MRP systems based on a large survey of MRP users. These users report they have achieved significant improvements in inventory turnover, delivery performance, and other benefits, and that further improvements are expected when their MRP systems are fully implemented. A series of regression models are also described in the paper which explain why some companies achieve more benefits from MRP than others. The models are not only fitted to the sample data, but they obtain good predictive results on a holdout sample. These models indicate that companies should stress a broad approach to implementation; there is no one overriding factor which guarantees MRP success.