Blood samples were obtained from 237 consecutive injured patients before any resuscitation. Microaggregate (MA) formation was measured by the screen filtration pressure (SFP) technique. Arterial blood gases were analyzed concomitantly and the respiratory index (RI) calculated. An Injury Severity Score (ISS) was calculated for each patient. The results show a significantly larger amount of trauma in the patients with the higher SFP results (p< 0.001). Associated with the increase in the amount of MA formation was a significant decrease in arterial pO2(p< 0.05) and a significant rise in the RI (p< 0.005), suggesting a correlation between MA formation and pulmonary impairment. Attempts to prevent or resolve MA formation may result in a decrease in the incidence or severity of post-traumatic pulmonary insufficiency.