The Metropolitan Regional Information Systems, Inc. (known as MRIS) is the largest Multiple Listing Service in the United States. As of late February 2007, it serves over 59,300 real estate professionals in Baltimore-Washington Metropolitan Area including Maryland, Washington DC, Northern Virginia, and parts of West Virginia and Pennsylvania.

Currently this is the largest MLS in North America, and it has evolved from a county-based system controlled by individual Assocations of Realtors operating as members of and under the rules of the National Association of Realtors. By the early 1990s it became apparent that a sophisticated computer-based system was necessary to integrate the MLS systems of the many counties and jurisdictions in the tri-state areas (or, at least, tri-jurisdictions) of Maryland, Washington, DC and Virginia, most of which were in close proximity to the capital.

Originally, many county Associations offered dumb-terminal access to the MLS only in brokers' offices at speeds such as 300 baud and, eventually by the late 1980s, at 1,200 baud. After being founded in 1992, MRIS operated as a computer-based program on the Windows 3.1 operating system, with proprietary software issued by MRIS. (This was not unlike the plethora of internet access companies, each with its own software, which emerged at that time). Since updates required a new software release via floppy disc to be installed by all subscribers, changes came slowly.

It was only when MRIS decided to become an internet-based system with access available to all brokers/agents via the internet from any computer anywhere, that the present system was created.