The aim of this study was to assess the contribution of bronchoalveolar lavage (BAL) in the diagnosis of fat embolism syndrome (FES). The presence of fat droplets in alveolar macrophages was addressed in 13 trauma patients with bone fractures and 10 non-trauma patients with acute respiratory distress syndrome (ARDS). The control group was composed of 5 anesthesized patients with ischemic heart disease, immediately prior to cardiac surgery. Two patients with suggestive clinical and laboratory signs of FES had 40% and 24% fat-containing alveolar cells, respectively. The trauma patients without signs of FES displayed a wide variation in the percentage of fat-containing macrophages (from 3% to 95%). Most of the patients with ARDS who were receiving lipid emulsion as part of their parenteral nutrition, had a high percentage (> 85%) of fat-containing macrophages. Patients with normal lungs had no fat-containing macrophages. Our findings suggest that BAL Oil Red O-positive macrophages are frequently observed in trauma patients irrespective of the presence of FES. Therefore, estimation of the percentage of fat-containing macrophages from BAL is an unreliable marker of FES.