- 18p-
- 1972 outbreak of smallpox in Yugoslavia
- 1984 Moroccan radiation accident
- 2006 dengue outbreak in India
- 2009 flu pandemic in Costa Rica
- 2009 Flu pandemic in North America
- 2009 Papua New Guinea cholera outbreak
- 3 hydroxyisobutyric aciduria
- 3-hydroxy-3-methylglutaryl-CoA lyase deficiency
- 3-hydroxyacyl-coenzyme A dehydrogenase deficiency
- A-scan ultrasound biometry
- Ioseb Abakelia
- Abasia
- ABCD rating
- Abderhalden reaction
- Abdominal mass
- Abdominal x-ray
- Abductor wedge
- Abel Salazar Biomedical Sciences Institute
- Neva Abelson
- Absorptive state
- Academia Nacional de Medicina
- Academy of Applied Pharmaceutical Sciences
- Academy of Medical Sciences, United Kingdom
- Acanthocephalus (animal)
- Acanthocheilonemiasis
- Accessible image
- Bojan Accetto
- Accommodative insufficiency
- ACET (AIDS charity)
- Achard syndrome
- Achard–Thiers syndrome
- Achondrogenesis type 2
- Acid lipase disease
- Acid perfusion test
- Acid Survivors Trust International
- Acid-citrate-dextrose
- ACIMED
- Johann Christian Gottlieb Ackermann
- ACP Smart Medicine
- Acquired disorder
- Acral keratoderma
- Acral necrosis
- Acrofrontofacionasal dysostosis
- Acrokeratoelastoidosis
- Acromicric dysplasia
- Acromioplasty
- Acropustulosis
- Action study
- Activated PI3K Delta Syndrome
- Active chromatin sequence
- Active immunization
- Acute eosinophilic leukemia
- Acute kidney injury
- Acute myeloblastic leukemia
- Acute myeloid dendritic cell leukemia
- Acute uric acid nephropathy
- James Makittrick Adair
- Adams–Nance syndrome
- Adaptive immune system
- Adelmota of Carrara
- Adenomere
- Adenosarcoma
- Adenosis
- Adenosquamous carcinoma
- Adipose tissue neoplasm
- Admission, discharge, and transfer system
- Adrenal adenoma
- Adrenergic bronchodilator
- Adrenocortical hyperfunction
- Advanced Diabetes Management Certification
- Advanced Medical Priority Dispatch System
- Adynamia
- Aegyptiacum
- Aesthetic Surgery Journal
- Affected community
- Arvid Afzelius
- Agent Orange
- Agoraphobia
- Luis Agote
- Aichi Medical College for Physical and Occupational Therapy
- Aichi Medical University
- AIDS education and training centers
- Air Evac
- Mark Akenside
- Alastrim
- Albert Szent-Györgyi Medical University
- Album graecum
- Alcarraza
- Alejandro Próspero Révérend
- Algesia
- Allergic inflammation
- Allergic response
- Allis clamp
- Allograft diseases
- Almeida–Pineda recurrent backpropagation
- Alphos
- ALPSA lesion
- Altru Health System
- Alveolar hydatid disease
- Alzheimer Disease and Associated Disorders
- Alzheimer's disease organizations
- Amastia
- Ambe
- Ambulance
- Ambulance Care Assistant
- Ambulance services of Victoria
- Ambulatory Payment Classification
- Ameloblastic fibroma
- American Academy of Craniofacial Pain
- American Academy of Optometry
- American Ambulance Association
- American Association for Women Radiologists
- American Association of Colleges of Podiatric Medicine
- American Association of Diabetes Educators
- American Board of Preventive Medicine
- American College of Pathology
- American College of Surgeons Oncology Group
- American Gastroenterological Association
- American Group Psychotherapy Association
- American Health Care Association
- American Horticultural Therapy Association
- American Joint Committee on Cancer
- American Journal of Emergency Medicine
- American Journal of Lifestyle Medicine
- American Journal of Medical Genetics
- American Journal of Obstetrics and Gynecology
- American Society for Artificial Internal Organs
- American Society for Gastrointestinal Endoscopy
- American Society of Gene Therapy
- American Society of Lymphology
- American Society of Registered Nurses
- American Telemedicine Association
- Amino acid transport disorder
- Aminoaciduria
- Ampicillin/flucloxacillin
- AmpliChip CYP450 Test
- Amsler grid
- Amsler sign
- Amyloid cardiomyopathy
- Amyotrophy
- Anaplastology
- Anatomical pathology
- Anatomical terminology
- Anatomical theatre
- Anchoring fibrils
- Ancylostoma
- Ancylostoma tubaeforme
- Ancylostomiasis
- John Anderson (genealogist, 1789–1832)
- Androgen suppression
- Androgen-induced hermaphroditism
- Anemostat
- Anesthesia cart
- Angina (disambiguation)
- Angiostasis
- Angiozyme
- Animal allergy
- Animal testing
- Anisochromasia
- Ankylosing hyperostosis
- Annals of Indian Academy of Neurology
- Annals of Thoracic Medicine
- Annuloaortic ectasia
- Anopsia
- Anorchia
- Anorectal anomalies
- Anoscopy
- Anoxia
- Anterior corticospinal tract
- Anterior cruciate ligament injury
- Anterior external arcuate fibers
- Anterior funiculus
- Anterior horn disease
- Anterior interosseous nerve
- Anterior median fissure of spinal cord
- Anterior median fissure of the medulla oblongata
- Anterior proper fasciculus
- Anterior ramus of spinal nerve
- Anterior triangle of the neck
- Anterior urethral cancer