'When you hear hooves, think zebras, not horses'; two challenging cases of interstitial lung disease (ILD).

Our case series describes two siblings with complex fibrosing lung diseases. The first patient was initially given a diagnosis of sarcoidosis based on imaging and exclusion of alternative diagnoses. A number of years after diagnosis, he had rapid deterioration of his disease and following surgical lung biopsy, his lung fibrosis was re-classified as chronic hypersensitivity pneumonitis (cHP) with a usual interstitial pneumonia pattern. He subsequently underwent successful lung transplantation. The second patient presented with rapidly progressing exertional dyspnoea. His bloods, imaging, bronchoalveolar lavage and histology were discussed at our multidisciplinary team meeting. His histology was most in keeping with subacute on cHP with overlapping imaging features between the two siblings. He was treated accordingly but unfortunately succumbed to his illness shortly after diagnosis. These cases highlight the difficulties differentiating between the various interstitial lung disease (ILD) subtypes and the challenges in management while also increasing awareness of familial ILD.