Rapidly fatal pulmonary tumour embolism is a rare complication of malignancy, and often presents as progressive dyspnea without obvious cause. We describe two cases presenting with a dramatic clinical picture of lactic acidosis and cardiopulmonary arrest soon after admission on ICU. The first patient was a 29-year old woman with a breast cancer seeming in remission who was admitted with rapidly increasing dyspnea since two weeks. The second patient was a 46-year old woman with HIV and no history of malignancy, who developed dyspnea and lactic acidosis over the course of a few days while she was investigated for an occipital brain lesion. Both patients died soon after admission and massive tumour emboli were found on autopsy. Breast cancer was the origin of the emboli in both cases. Symptoms were out of proportion to the initial physical cardiopulmonary findings and radiographic features. Clinical signs of pulmonary tumour embolism are non-specific and subacute. Prognosis is poor and definite diagnosis is usually made post-mortem. Solid malignancies such as breast cancer account for most of the cases. Pulmonary tumour embolism should be considered in critically ill patients with unexplained hypoxemia and lactic acidosis, mild or no radiological abnormalities and fast clinical deterioration. It may occur in young patients and in patients without history of malignancy.