We report on a 36-year-old man who presented to the emergency department with haemoptysis. Computed tomography (CT) of the thorax showed a pulmonary mass paramediastinal in the right upper lobe, with the density of a haematoma. Laboratory data demonstrated an absolute lymphocytosis of 5.900 × 109/l (normal range, 1.150–3.250 × 109/l) and a prolonged activated partial thromboplastin time (APTT) of 47.7 s (normal range, 28.0–39.0 s). A de novo diagnosis of lymphoplasmacytic lymphoma (Waldenström macroglobulinaemia) was made, complicated by an acquired von Willebrand syndrome (aVWS) as demonstrated by further laboratory investigations. In this case report, we present a case of aVWS with markedly prolonged APTT and haemoptysis that revealed an underlying Waldenström macroglobulinaemia.