A previously healthy woman in her early 20s presented in week 38 of pregnancy with a 2-week history of haemoptysis, positional dyspnoea and an examination suggestive of pulmonary oedema. A chest CT showed a left lower lobe lung mass invading through the inferior pulmonary vein into the left atrium and abutting the posterior mitral leaflet. Pulmonary oedema consequent on left inferior pulmonary vein occlusion was initially restricted to the left lower lobe, but progressed to involve the contralateral lung following extension of the mass to the mitral orifice, resulting in functional mitral stenosis. A biopsy was positive for poorly differentiated spindle cell neoplasm. This is the first reported case of a pregnant patient with spindle cell neoplasm of lung and of a spindle neoplasm causing mitral stenosis, as well as the youngest known case of this disease.