| 1 | MDD | MDA Airlines, Joint-Stock Company | Callsign: Donlines, | RUS | AVCODES |
| 2 | MDD | MDLR Airlines Pvt. Ltd. | IATA: 9Y, | IND | AVCODES |
| 3 | MDD | Air Maryland | Callsign: Air Maryland, | USA | AVCODES |
| 4 | MDD | | | USA | TRAFFIC |
| 5 | MDDA | MUEBLES DE DISENO ANDALUCIA | | | SCAC4 |
| 6 | MDDB | MICROBRASS | | | SCAC4 |
| 7 | MDDE | Maryland and Delaware R.R. Co. | added 1/1978; 1/1980; eliminated 10/1987; 1/1994-7/2005; eliminated 10/2009 | | NAKINA |
| 8 | MDDE | Maryland and Delaware R.R. Co. | added 1/78; 1/80; eliminated 10/87; 1/94-7/2000 | | NRHS |
| 9 | MDDE | MARYLAND & DELAWARE RAILROAD CO. | | | PWRC |
| 10 | MDDE | Maryland and Delaware Railroad | | | RSRV |
| 11 | MDDH | M & D TRANSPORTATION LLC | | | SCAC4 |
| 12 | MDDK | MADDOCK, ROGER, TRANSPORTATION | | | SCAC1 |
| 13 | MDDL | MANDERS DIESEL REPAIR INC | | | SCAC4 |
| 14 | MDDM | M & D LLC | | | SCAC4 |
| 15 | MDDN | MADDEN OIL COMPANY | | | SCAC4 |
| 16 | MDDO | METAL DYNAMICS DETROIT LLC | | | SCAC4 |
| 17 | MDDR | MIDLAND DIRECT INC | | | SCAC4 |
| 18 | MDDX | Old Line Holding Co., Inc. | 7/2000; eliminated 10/2004; 7/2005 (not listed) | | NAKINA |
| 19 | MDDX | Old Line Holding Co., Inc. | Jul-00 | | NRHS |
| 20 | MDDX | OLD LINE HOLDINGS COMPANY INC. | | | PWRC |
| 21 | MDDX | Old Line Holding | | | RSRV |
| 22 | MDDY | MITCHELL, D | | | SCAC4 |
