Tumor necrosis factor (TNF), rapidly becoming recognized as a mediator of inflammation, may be important in the pathogenesis of acute lung injury. Its role in the development of the adult respiratory distress syndrome (ARDS) in humans, however, has been difficult to clarify. To determine if TNF could be important early in the development of acute lung injury from multiple causes, we enrolled 103 patients within 8 h of meeting the criteria for an at-risk illness (sepsis, aspiration of gastric contents, severe pancreatitis, hypertransfusion, abdominal trauma, chest trauma, multiple fractures) and obtained multiple frequent blood samples for TNF measurements. Using five methods of TNF analysis, we were unable to find an association between TNF and the development of ARDS. However, we found significant differences in TNF measurements depending on the methods of analysis used, which could, at least in part, account for the inconsistencies in the published literature regarding the relationship between TNF and disease processes.