The increased susceptibility of severely injured patients to infection and death from sepsis has been attributed to abnormalities in cell-mediated immunity. The authors therefore assessed the relative number of peripheral blood T helper cells and T suppressor/cytotoxic cells and total T lymphocytes identified by the monoclonal antibodies (McA) OKT4, OKT8, and OKT3, respectively, in 25 patients with burns from 5 to 85% total body surface area (TBSA) (mean: 40%) and 21 patients with nonthermal injuries (mean Injury Severity Score (ISS): 21.4). Patients were compared to 21 healthy controls. Cells reacting with the McA were detected by flow cytometry, which enabled the examination of a population of cells the size of T lymphocytes, excluding larger contaminating cells that might bind the McA. Patients with burns of 30% TBSA or greater had a significant reduction (p less than or equal to 0.05) in OKT3+ cells up to 50 days post-burn. Both septic and nonseptic burn patients had reduced numbers of OKT3+ cells, as did patients after nonthermal injury, suggesting that this reduction was due to the injury itself. Patients with smaller burns (less than 30% TBSA) as a group did not have reduced OKT4+ cells, whereas those with larger burns showed significant reductions in OKT4+ cells (P less than or equal to 0.05) at 0 to 5, 6 to 10, 11 to 20, 21 to 30, and 41 to 50 days post-burn. Seven burn patients who became septic 10 days post-burn or later had significantly lower OKT4+ cells within 10 days of injury (mean: 33.75% +/- 7.4 SEM) than 10 patients who remained free of sepsis (mean: 42.2% +/- 5.4, p = 0.004). Patients with uncomplicated nonthermal injuries failed to show any significant reduction in OKT4+ cells. Following thermal injury, a reduction in OKT8+ cells was observed up to 10 days in patients with burns less than 30% TBSA, and up to 20 days in patients with larger burns. In both groups, at no time were increased OKT8+ cells found to correlate with clinical events. In patients with nonthermal injury, OKT8+ cells generally remained near the normal range.