MCI or mci may refer to:

• MCI Communications, originally Microwave Communications Inc., a corporation that operated as MCI from 1963 to 1998
  • MCI Inc., formerly called WorldCom, which acquired MCI Communications, and was later acquired by Verizon Communications
• MCI (recording studio gear), a manufacturer of multi-track audio tape recorders and mixing consoles for professional studio recording (officially Music Center Incorporated, later Music Consultants Incorporated)
• Mobile Telecommunication Company of Iran, the largest mobile phone operator in Iran
• Motor Coach Industries, a coach/bus manufacturing company
• Music Collection International, the music division of Video Collection International
• MCI, Frank Farian's record label

• Maine Central Institute, a boarding school in Maine, United States
• Malvern Collegiate Institute, a secondary school in Toronto, Ontario, Canada
• Marine Corps Institute, a coursework of Marine Corps education and training programs
• Martingrove Collegiate Institute, a secondary school in Toronto, Ontario, Canada
• MCI Management Center Innsbruck, a privately organized business school in Innsbruck, Austria
• Medical Council of India, a former statutory body for establishing uniform and high standards of medical education in India
• Mennonite Collegiate Institute, a private religious school in Gretna, Southern Manitoba
• Middlefield Collegiate Institute, a secondary school in Markham, Ontario, Canada
• Museum Conservation Institute, of the Smithsonian Institution

• Ministry of Communications and Information, a ministry of the Government of Singapore
• Motek Cultural Initiative, a Toronto-based nonprofit organization promoting Israeli music and culture

• Millicurie (mCi), 1/1000 of a curie, a non-SI unit of radioactivity
• Megacurie (MCi), 1,000,000 times a curie, a non-SI unit of radioactivity
• Macroinvertebrate Community Index, an index where the presence or lack of macroinvertebrates is used for monitoring stream health in New Zealand
• Magnetic current imaging, using a scanning SQUID microscope
• Malicious caller identification, a type of enhanced telephone service
• Mass-casualty incident, a medical emergency involving more patients than can be easily handled by the crews initially assigned to the incident
• Media Control Interface, an API for controlling multimedia peripherals connected to a Microsoft Windows or OS/2 computer
• Meta-circular evaluator, in computing
• Methylchloroisothiazolinone, in chemistry
• Mild cognitive impairment, a neurological affliction often associated with Alzheimer's disease
• Modulation contrast imaging, a term related to optical transfer function

• MasterChef Indonesia, an Indonesian televised cooking competition

• 1101, in Roman numerals
• Kansas City International Airport (IATA code), Missouri, United States
• Meal, Combat, Individual ration, the successor to the U.S. Army C-ration
• Michigan City station (Amtrak code), Indiana, United States
• Miss Chinese International Pageant, an annual pageant for international women of Chinese descent
• Monetary conditions index, a macroeconomic index number relevant for monetary policy
• Manchester City F.C., abbreviated as MCI
• Mese language, an ISO 639-3 code