Recent studies suggest that an increase in fat-containing macrophages in bronchoalveolar lavage (BAL) fluid may be helpful in the diagnosis of fat embolism syndrome (FES). Nevertheless, none of these studies have explored the sequential findings of BAL fluid. We report the case of a 19-year-old man admitted to our intensive care unit because of dyspnea with radiographic evidence of bilateral alveolar infiltrate after traumatic fracture. Analysis of BAL fluid on the third hospital day revealed 8.3% fat-containing macrophages and a lipid-laden macrophages (LLM) index of 23. Pathologic examination of lung biopsy showed numerous fat globules within arterioles. For comparison, the BAL fluid from four other patients with acute respiratory distress syndrome (ARDS) but without FES was also analyzed. The underlying diseases leading to ARDS included Wegener's granulomatosis in one case, pneumonia in two cases, and alveolar proteinosis in one case. The percentages of fat-containing macrophages in these specimens were 1.3%, 52%, 2.3%, and 74%, respectively. The LLM indexes were 1, 133, 3, and 243, respectively. As the patient's condition improved, the percentage of fat-containing macrophages in the BAL fluid decreased to 4.7% on the eighth hospital day and the LLM index also decreased to 6. These findings suggest that the presence of fat-containing macrophages in BAL fluid is not specific for the diagnosis of FES, but serial changes in the percentage of these cells and the LLM index may be helpful in the follow-up of disease severity.