Leukocyte chemotaxis in vitro was studied by a modification of the Boyden technic in 46 thermally injured patients. All patients demonstrated a decrease in leukocyte migration through a Nuclepore filter toward a standard casein-serum chemotactic agent. Leukocyte chemotaxis was inversely correlated with burn size during the first 72 h after injury. After 72 h, leukocyte chemotaxis directly correlated with clinical status and was highly predictive for ultimate mortality. Since mortality was largely due to infection, these findings suggest that suppression of leukocyte chemotaxis may explain the susceptibility to opportunistic infection in thermally injured patients.