Summary: GAO addressed the Air Force's plan to replace a portion of its KC-135 aerial refueling tanker fleet with leased Boeing 767 aircraft. Although the Air Force has a long term requirement to replace its aging fleet of KC-135 tankers, the urgency of the need in the short term is unclear. The Air Force stated that the leasing arrangement would allow it to acquire new tankers three years earlier than through its most recent procurement plan. This would allow the Air Force to retire old, less capable KC-135s, thus saving maintenance costs on those aircraft. Because the Air Force is still negotiating the lease details, it could not provide information on the cost effectiveness of leasing aircraft instead of purchasing them. Although GAO has not taken a position on the overall policy of leasing versus purchasing defense equipment, it found that, from a cost standpoint, leasing is more expensive in the long run. Because the 767 aircraft is larger than the KC-135, there will be some infrastructure improvement costs, such as for building or modifying hangars, taxiways, and runway aprons. Additional costs would likely include simulators and project management. The depots have undertaken some measures to speed up KC-135 maintenance and repair times; however, the extent to which these actions are helping or whether other measures could be taken is unknown.