Severe immunosuppression occurs after major thermal burns, accidental injuries, and extensive surgical operations, and probably contributes substantially to patient morbidity and mortality. The mechanism of immunosuppression is unknown and attempts to explain it are contradictory. The contradictions can be resolved by assuming the activation by injury of the suppressor T-cell system, which is involved in normal immunoregulation. If this is true, then generalised, non-specific attempts to bolster the immune response in these patients may be inappropriate.