Burn death based on circulatory shock is often encountered after recovery from primary shock in patients with deep and extensive burns, i.e., late death. Several toxic substances have been proposed, however, the responsible substance remains obscure. Since we have found leukotoxin, a highly cytotoxic linoleate epoxide biosynthesized by neutrophils, in the burned skin, in the present study we determined plasma leukotoxin concentrations in various degree of 30 burn patients. C-reactive protein and circulatory white blood cells were also measured. A significantly high mortality rate of patients with extensive burns (burn surface area over 70%) was observed compared with that in patients with burn surface area under 70%, and significantly high leukotoxin concentrations were observed within a week, and 3 weeks after the thermal injury in patients with extensive burns compared with those in patients with burn surface area under 70%. There were two peaks of plasma leukotoxin concentrations, i.e., the early phase (within 1 week) and the late phase (over 1 week) in patients with extensive burns. Plasma leukotoxin concentrations significantly correlated with burn surface area in the early phase, and similar correlations were observed in the late phase. A significantly high mortality rate (61%) of patients with peak leukotoxin concentrations over 30 nmol/ml was observed compared with 8% for those below 30 nmol/ml. Plasma leukotoxin concentration correlated significantly to C-reactive protein concentration, log (leukotoxin nmol/ml) = 0.042 x C-reactive protein (mg/dl) + 0.74, (r = 0.83, P < 0.01) in the late phase. From these results, it is concluded that leukotoxin is produced in patients with burns particularly in the late phase of extensive burns, and leukotoxin might play an important role in the tissue destructive procedure associated with severe burns.