Chronic myelomonocytic leukaemia is an atypical myeloproliferative disorder with a natural history of progression to acute myeloid leukaemia, a complex and poorly understood response by the bone marrow to stress. Cardiac surgery activates many inflammatory cascades and may precipitate a systemic inflammatory response syndrome. We present a case of undiagnosed chronic myelomonocytic leukaemia who developed rapidly fatal multi-organ dysfunction following cardiac surgery due to an acute leukaemoid reaction.