- MCI Worldcom , Inc., a former American telecommunications company acquired by Verizon Communications
- Microwave Communications Inc., later MCI Communications , the corporation that operated as MCI from 1963–1998
- The airport code for the Kansas City International Airport in Kansas City, Missouri, USA
- Maine Central Institute , a boarding school in Maine, founded in 1866
- Martingrove Collegiate Institute , a high school located in Etobicoke, Toronto, Canada
- Mild Cognitive Impairment , a neurological affliction often erroneously connected to Alzheimer's Disease
- Motor Coach Industries , a company that builds buses
- MCI (audio) , a company that manufactured multi-track audiotape recorders for professional studio recording
- Massachusetts Correctional Institution, a list of which can be found at List Of Massachusetts State Prisons
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