| Abbreviations | Meaning |
 | LIS | Locked-in Syndrome |
 | LK | Lamellar Keratoplasty; Landry-Kussmaul [syndrome]; Left Kidney; Lichenoid Keratosis; Lymphokine |
 | LKS | Landau-Kleffner Syndrome; Liver, Kidneys, Spleen |
 | LLS | Lazy Leukocyte Syndrome |
 | LMB | Laurence-Moon-Biedl [syndrome]; Left Main Bronchus; Leiomyoblastoma; Leukomethylene Blue |
 | LMBB | Laurence-Moon-Bardet-Biedl [syndrome] |
 | LMBS | Laurence-Moon-Biedl Syndrome |
 | LMPS | Lethal Multiple Pterygium Syndrome |
 | LMS | Lateral Medullary Syndrome |
 | LN | Lesch-Nyhan [syndrome] |
 | LNKS | Low Natural Killer Syndrome |
 | LNS | Lateral Nuclear Stratum; Lesch-Nyhan Syndrome |
 | LOCS | Laryngoonychocutaneous Syndrome |
 | LOGIC | Laryngeal And Ocular Granulations In Children Of Indian Subcontinent [syndrome] |
 | LOS | Low Cardiac Output Syndrome |
 | LPHAS | Limb/pelvis-hypoplasia/aplasia Syndrome |
 | LQT | Long QT [syndrome] |
 | LQTS | Long QT Syndrome |
 | LRS | Lateral Recess Syndrome |
 | LRS | Lumboradicular Syndrome |
 | LRSS | Late Respiratory Systemic Syndrome |
 | LS | Leigh Syndrome |
 | LS | Leriche Syndrome |
 | LUFS | Luteinized Unruptured Follicle Syndrome |
 | LWS | Lowry-Wood Syndrome |
 | LYES | Liver Yang Exuberance Syndrome |
 | LYSLK | Lymphoma Syndrome Leukemia |
 | MA | Martin-Albright [syndrome] |
 | MACS | Multicenter Acquired Immunodeficiency Syndrome (AIDS) Cohort Study |
 | MAGIC | Mouth (or Mucosal) And Genital Ulceration With Inflamed Cartilage [syndrome] |
 | MAIDS | Mouse Acquired Immunodeficiency Syndrome |
 | MAIS | Manual Axial In-line Stabilization; Mild Androgen Insensitivity Syndrome |
 | MAS | Macrophage Activation Syndrome |
 | MAS | McCune-Albright Syndrome |
 | MAS | Meconium Aspiration Syndrome |
 | MAS | Milk-alkali Syndrome |
 | MAS | Morgagni-Adams-Stokes [syndrome] |
 | MASA | Medical Association Of South Africa; Mental Retardation-aphasia-shuffling Gait-adducted Thumbs [syndrome]; Mutant Allele Specific Amplification |
 | MBPS | Megabits Per Second; Myeloblastic Syndrome |
 | MBS | Marie-Bamberger Syndrome |
 | MBS | Martin-Bell Syndrome; Media Broadband Service |
 | MCA/MR | Multiple Congenital Anomaly/ Mental Retardation [syndrome] |
 | MCAS | Middle Cerebral Artery Syndrome |
 | MCES | Medical Care Evaluation Study; Multiple Cholesterol Emboli Syndrome |
 | MCINS | Minimal Change Idiopathic Nephrotic Syndrome |
 | MCLNS, MCLS | Mucocutaneous Lymph Node Syndrome |
 | MCNS | Minimal Change Nephrotic Syndrome |
 | MCS | Malignant Carcinoid Syndrome |
 | MCS | Miles-Carpenter Syndrome |
 | MDCR | Miller-Dieker [syndrome] Chromosome Region |