Adenosquamous carcinoma arising within a long-standing intrapulmonary bronchogenic cyst in an adult presenting with hyponatraemia.

A 74-year-old woman's persistent hyponatraemia led to the discovery of an adenosquamous carcinoma within an intrapulmonary bronchogenic cyst (IPBC), diagnosed 59 years prior. This is the first reported case of such a transformation in an IPBC. An adenosquamous carcinoma, originating from an intrapulmonary bronchogenic cyst identified 59 years prior, was discovered during the workup for a patient's unexplained, persistent hyponatraemia.