Interstitial disease is a pathological process involving the interlobular connective tissue (i.e. scaffolding of the lung).

Differential diagnosis of interstitial disease includes:
1. Pulmonary edema (which can move into the alveoli)
2. Collagen disease (fibrosis)
3. Sarcoidosis
4. Pneumoconiosis
5. Metastatic disease (from the lymphatics)
6. Inflammatory conditions (viral/interstitial pneumonia)