A cryptogenic disease is a disease of which the cause is unknown. It may be used in a particular case, when the nature of the patient's condition is known but the cause has not been found (e.g. cryptogenic stroke). The word "cryptogenic" also appears in the names of some disease entities, when the situation is sufficiently common to be considered a diagnosis in its own right (e.g. cryptogenic fibrosing alveolitis).
"Cryptogenic", "idiopathic" and "primary" may all be used in both these senses, but "cryptogenic" is more likely to be used where there is presumed to be a simple cause but this happens to have eluded discovery. "Cryptogenic" is used in this technical sense in the description of epilepsy syndromes (although the distinction has now been officially abandoned).
In practice, the term "cryptogenic" is largely restricted to certain specific conditions. These include:
- diseases of the lung: cryptogenic fibrosing alveolitis, cryptogenic organizing pneumonia
- diseases of the liver: cryptogenic cirrhosis, cryptogenic hepatitis
- diseases of the brain: cryptogenic stroke, cryptogenic epilepsy