The report describes a cost study performed for the new replacement all weather instrument landing system. Costs are determined for two Scenarios: A - following the National Plan for Microwave Landing Systems, and B - those currently operational systems (GCA - ILS ...) expected to exist in the absence of the national MLS. For civil ground systems, Scenario A (MLS) produces a significant cost saving; resulting principally from the elimination of restrictive site requirements and reduced frequency of flight inspections. Military ground systems also favor MLS due to earlier retirement of GCA systems. Both of these cost savings are offset by the extra cost of airborne MLS, especially to the more than 700,000 general aviation aircraft projected for the year 2000. Over a 30-year period, the total costs to the nation under the MLS plan are not significantly higher than costs of continuing with current systems. The conclusion is that the lack of a cost penalty, combined with the performance, safety, and economic benefits, make a solid case for MLS. (Author).