This paper assesses the impact of a decision support system (DSS), the Investment Simulation Model (ISM), on recommendations given by extension officers to pig farmers. ISM gives insight into the relation between strategic investment decision rules and their simulated outcome. An evaluation procedure was developed for testing ISM under operational use. The model was put to the test on three farms and with three extension officers. For each farm, three sets of recommendations were prepared: (1) without using ISM, based on a farm visit to collect data; (2) with use of ISM, based on a farm visit to collect data; and (3) with use of ISM, but with data collected by another advisor. The field test resulted in striking differences between recommendations based on ISM and those not based on ISM. In the latter recommendations, immediate farm expansion was regarded as not feasible for two of the three farms. When using ISM, a farm expansion strategy was advised for all three farms, however, not immediately. Without using ISM, replacement strategies for the buildings, manure costs, and family expenditures were largely ignored in the advice, contrary to the advice based on ISM. Adjusted for the difference in travelling time, the time required for preparing advice using ISM equaled 80% of the time required for preparing advice not using ISM. Whether or not the extension officer himself collected the farm data did not influence the advice.