- Areflexia 796.1
- Areola - see condition
- Argentaffinoma (M8241/1) - see also Neoplasm, by site, uncertain behavior
- Argentinian hemorrhagic fever 078.7
- Arginosuccinicaciduria 270.6
- Argonz-Del Castillo syndrome (nonpuerperal galactorrhea and amenorrhea) 253.1
- Argyll-Robertson phenomenon, pupil, or syndrome (syphilitic) 094.89
- Argyria, argyriasis NEC 985.8
- Arhinencephaly 742.2
- Arias-Stella phenomenon 621.30
- Ariboflavinosis 266.0
- Arizona enteritis 008.1
- Arm - see condition
- Armenian disease 277.31
- Arnold-Chiari obstruction or syndrome (see also Spina bifida) 741.0
- Arousals
- Arrest, arrested
- Arrhenoblastoma (M8630/1)
- Arrhinencephaly 742.2
- Arrhythmia (auricle) (cardiac) (cordis) (gallop rhythm) (juvenile) (nodal) (reflex) (sinus) (supraventricular) (transitory) (ventricle) 427.9
- Arrillaga-Ayerza syndrome (pulmonary artery sclerosis with pulmonary hypertension) 416.0
- Arsenical