Red blood cells (RBC) have been shown to become less deformable during infection. The RBC deformability index (DI) was measured within 24 hours of admission in 37 patients who had suffered trauma and every 48 to 72 hours thereafter while they were in the surgical intensive care unit to assess whether DI could be used as an early indicator of infection after injury. Infection was defined as a temperature of 101 degrees F or more and a white blood cell count of more than 12,000/cm3 associated with a positive culture. Eighteen patients developed an infection, and 19 patients did not. On day 1, both groups showed a significant decrease in DI, compared to controls (0.33 +/- 0.18 and 0.34 +/- 0.25 for patients with infection and patients with no infection vs 1.52 +/- 0.12 for control volunteers; p less than 0.05). In the group with no infection, the DI improved in 16 of 19 patients after injury; the DI in patients with infection continued to decrease in 17 of 18 patients. The decrease in DI occurred 4 +/- 2 days (range, 2 to 8 days) before the diagnosis of infection. No significant differences were apparent in the absolute white blood cell count between the group with infection and the group with no infection at any time after injury. Differences in maximal temperature were noted on day 3 and beyond; however, 30% of patients with no infection had a temperature of more than 101 degrees F for 7 days. These data show that trauma results in a significant decrease in RBC deformability and that serial changes in DI appeared to predict which patients would develop an infection and which patients would recover uneventfully. RBC deformability may be helpful in early detection of infection in patients who have suffered trauma.