- Arthrosis (deformans) (degenerative) (see also Osteoarthrosis) 715.9
- Arthus phenomenon 995.21
- Articular - see also condition
- Articulation
- Artificial
- ARV (disease) (illness) (infection) - see Human immunodeficiency virus (disease) (illness) (infection)
- Arytenoid - see condition
- Asbestosis (occupational) 501
- Asboe-Hansen's disease (incontinentia pigmenti) 757.33
- ASC-H (atypical squamous cells cannot exclude high grade squamous intraepithelial lesion) 795.02
- ASC-US (atypical squamous cells of undetermined significance) 795.01
- Ascariasis (intestinal) (lung) 127.0
- Ascaridiasis 127.0
- Ascaridosis 127.0
- Ascaris 127.0
- Ascending - see condition
- Aschoff's bodies (see also Myocarditis, rheumatic) 398.0
- Ascites 789.59
- Ascorbic acid (vitamin C) deficiency (scurvy) 267
- ASCVD (arteriosclerotic cardiovascular disease) 429.2
- Aseptic - see condition
- Asherman's syndrome 621.5