Serum fibronectin (Fn) level was serially determined in nineteen septic patients. Fn concentration was found significantly decreased in septic patients (mean +/- SE, 120 +/- 11 micrograms/ml) as compared to the control (205 +/- 11 micrograms/ml). The decrease of Fn was correlated with the severity of sepsis; it was persistent and more important in severe septic cases. The lowest Fn concentration was found in nonsurviving patients (78 +/- 18 micrograms/ml). Serum Fn level may reflect the degree of organ failure and predict the evolution of sepsis.