Background: The reason for increased infectious complications after traumatic injury is complex and incompletely understood. We propose a relationship between the energetic state of circulating immune cells and immune cell function in traumatic injury. To examine this relationship, cellular adenosine triphosphate (ATP) concentration and cellular functions were measured in peripheral blood mononuclear cells from trauma patients and normal subjects. Materials and Methods: Mononuclear cells were isolated within 24 h of injury from trauma patients without (n = 12) or with (n = 10) hypotension (groups 1 and 2, respectively), and a group of normal control subjects (group 3, n = 13). Mononuclear cells were assayed for ATP levels using bioluminescence. Phagocytosis was quantified via flow cytometry after ingestion of fluorescent microspheres and phagocytic index (PI) was calculated (average number of particles ingested per monocyte). Protein synthesis was quantified using incorporation of 35S-labeled methionine into cultured cells. Comparisons between groups were performed using one-way analysis of variance (ANOVA) with adjustment for multiple comparisons. Results: All but one trauma patient suffered blunt injury. There were significantly more transfusions in group 2 (hypotensive) patients (p = 0.0005). Nosocomial infections, length of stay (LOS), and mortality did not differ between groups 1 and 2. Concentrations of ATP and PI in both groups of trauma patients did not significantly differ from controls. In hypotensive trauma patients, increased duration of hypotension was associated with increased mononuclear cell ATP levels (r2 = 0.227). A negative correlation between PI and ATP levels in trauma patients was discovered. Incorporation of 35S was significantly greater in normotensive trauma patients than controls. Conclusion: Mononuclear cell ATP levels and measured functions are preserved in early traumatic injury.