In contrast to pulmonary parenchyma metastases or lymphangitic carcinomatosis, neoplastic emboli of small pulmonary arteries and capillaries frequently go unrecognized and are only discovered at autopsy. Five patients (48 +/- 12 years old) were admitted to 3 intensive care units for severe acute respiratory failure and died between the first and the tenth day following hospitalization. Each patient had a history of rapidly progressive dyspnea, and physical examination showed clinical evidence of right ventricular failure. The lungs were clear on chest X-rays and the ECG revealed sinus tachycardia with a right QRS axis. The mean partial pressures of oxygen (PaO2) and carbon dioxide (PaCO2) were, respectively, 50.8 +/- 9.1 mm Hg and 22.2 +/- 2.4 mm Hg. A swan-Ganz catheter, inserted into 4 patients, revealed pulmonary arterial hypertension (55, 43, 37, 28) with capillary wedge pressure within the normal limits and cardiac output normal or low (3.0, 3.8, 4.4, 5.0 l/min). Pulmonary angiograms from each patient showed decreased distal lung perfusion without any proximal defects suggestive of pulmonary embolism. The inferior vena cava always appeared clear. Malignant cells were found upon autopsy (4 cases) in the lumina of the pulmonary arterioles and the primary site of the cancer was determined in 3 patients (2 hepatomas and 1 pancreatic carcinoma). The last patient had a known breast cancer with bone marrow metastases and clinical, hemodynamic and angiographic evidence of neoplastic emboli. The clinical course of neoplastic emboli can suggest acute pulmonary embolism, but the diagnosis can only be advanced after pulmonary angiography, especially if the patient is to have a cancer.(ABSTRACT TRUNCATED AT 250 WORDS)