This is a list of diseases starting with the letter "J".

• Jackson–Weiss syndrome
• Jacobsen syndrome
• Jadassohn–Lewandowsky syndrome
• Jaffer–Beighton syndrome
• Jalili syndrome
• Jancar syndrome
• Jankovic–Rivera syndrome
• Jansen type metaphyseal chondrodysplasia
• Jansky–Bielschowsky disease
• Japanese encephalitis
• Jarcho–Levin syndrome
• Jejunal atresia
• Jensen syndrome
• Jequier–Kozlowski skeletal dysplasia
• Jervell and Lange-Nielsen syndrome
• Jeune syndrome situs inversus
• Jeune syndrome
• Jeune asphyxiating thoracic dystrophy

• Job syndrome
• Johanson–Blizzard syndrome
• Johnson–Hall–Krous syndrome
• Johnson–Munson syndrome
• Johnston–Aarons–Schelley syndrome
• Jones–Hersh–Yusk syndrome
• Jones syndrome
• Jorgenson–Lenz syndrome
• Joseph disease
• Joubert syndrome bilateral chorioretinal coloboma
• Joubert syndrome

• Juberg–Hayward syndrome
• Juberg–Marsidi syndrome
• Judge–Misch–Wright syndrome
• Jumping Frenchmen of Maine
• Jung–Wolff–Back–Stahl syndrome
• Juvenile cataract cerebellar atrophy myopathy mental retardation
• Juvenile dermatomyositis
• Juvenile gastrointestinal polyposis
• Juvenile gout
• Juvenile hyaline fibromatosis
• Juvenile macular degeneration hypotrichosis
• Juvenile muscular atrophy of the distal upper limb
• Juvenile myoclonic epilepsy
• Juvenile nephronophthisis
• Juvenile rheumatoid arthritis
• Juvenile temporal arteritis
• Juliannite nephronophthisis