To find the diagnostic methods for subclinical stage fat embolism syndrome (FES), we established an experimental animal model, using fat intravenous injection. The fat was obtained from the long bone marrow cavity of homologous dogs. Fourteen healthy mongrel dogs received 0.7 ml/kg fluid marrow fat injection and all of them developed FES within 48 hours. The blood samples collected from the pulmonary vessels by floating catheter and peripheral vein at different time intervals were subjected to blood gas analysis and were frozen sectioned rapidly. The sections were stained with oil red 'O'. Positive result was seen 2 hours after fat injection in both pulmonary and peripheral blood. Computer image analysis showed that the number and diameter of fat droplets in pulmonary vascular blood were obviously higher and larger than those in peripheral vein blood. These findings were correlated well with blood gas changes and clinical features. The demonstration of fat droplets from pulmonary or peripheral blood by oil red 'O' staining combined with blood gas changes (PaO2 < 7.99 kPa, P(A-a)O2 > 6.09 kPa) may be rapid and specific for early diagnosis of FES. In the treatment of FES, dexamethason can stabilize the cellular membranes and inhibit the neutrophil response to fatty acid and the release of phospholipase A2, arachidonic acid and platelet aggregation.