- Labyrinthitis (dizziness and vertigo)
- Laceration (skin cut)
- Lactose intolerance
- Lacunar stroke
- Langerhans cell histiocytosis (Histiocytosis X)
- Large cell lung cancer
- Laryngeal carcinoma (throat cancer)
- Laryngitis (voice box inflammation)
- Latex allergy
- Laxative abuse
- Lazy eye (amblyopia)
- Lead overdose (lead poisoning)
- Lead poisoning (toxicity)
- Leg laceration (skin cut)
- Leg swelling (leg edema)
- Legg Calve Perthes disease (childhood hip condition)
- Legionellosis (Legionnaires' disease)
- Leishmaniasis (parasitic infection, kala azar)
- Lemierre's syndrome
- Leptospirosis (bacterial infection)
- Leukemia (blood cell cancer)
- Leukoplakia
- Lewy body dementia (common type of dementia)
- Lichen planus (chronic skin inflammation)
- Ligament sprain (connection between two bones)
- Ligament tear (connection between two bones)
- Limb ischemia (decreased blood flow)
- Lip biting
- Lip laceration (skin cut)
- Lip swelling
- Lipoma (benign fatty tumor)
- Listeria (serious bacterial illness)
- Livedo reticularis (purplish skin discoloration)
- Liver abscess (collection of pus)
- Liver cancer (tumor)
- Liver injury (liver trauma)
- Loffler's syndrome (pulmonary eosinophilia, lung inflammation)
- Long QT Syndrome (inherited heart condition)
- Low back pain (lumbago)
- Lou Gehrig's disease (amyotrophic lateral sclerosis, ALS)
- Low blood pressure (hypotension)
- Low platelets (thrombocytopenia)
- Lower leg fracture (tibia, fibula)
- Lumbar spinal stenosis (spinal cord compression)
- Ludwig’s angina (severe infection in the floor of the mouth and neck)
- Lumbar spondylosis (spine arthritis)
- Lung abscess (collection of pus)
- Lung cancer (tumor)
- Lyme disease (bacterial infection from ticks)
- Lupus (systemic lupus erythematosus, SLE)
- Lymphangitis (lymph tissue inflammation)
- Lymphadenitis
- Lymphedema (lymphatic obstruction)
- Lymphoma (lymph node cancer)