We present a case of constrictive pericarditis with concomitant blood and bone marrow appearances of chronic myelomonocytic leukaemia (CMML). Despite surgical treatment with pericardiectomy, the patient deteriorated into multiorgan failure. Pericardial histology disclosed a typical inflammatory picture with no evidence of monocytic or malignant infiltrate. Following intensive collaboration between cardiologists, haematologists and rheumatologists via daily email exchanges, a diagnosis was reached of autoinflammatory constrictive pericarditis with a non-infiltrative coexisting CMML. The key to achieving a rapid and sustained response was a trial of high-dose steroids followed by intravenous immunoglobulins. This achieved restoration of cardiac function, resolution of symptoms and near normalisation of inflammatory markers. A diagnosis of concurrent CMML was confirmed at 3 months. The patient remains well, taking colchicine and steroids.