In order to elucidate the intrinsic defects of inflammatory cells in adult respiratory distress syndrome (ARDS), 10 ARDS patients, 6 non-ARDS respiratory failure patients and 30 healthy controls were included in this study. The peripheral blood neutrophils (PBNs) were isolated from heparinized venous blood. Human alveolar neutrophils (HANs) were taken during bronchoalveolar lavage (BAL). Chemiluminescence was then measured with a luminometer (LKB Wallac) and antibody dependent cellular cytotoxicity (ADCC) was assayed using a Cr51 cytotoxic assay. The bacterial killing test utilized heparinized PBNs and Staphylococcus aureus. The results showed that the chemiluminescence of PBNs and HANs was higher in ARDS patients than in the normal control group (p < 0.05). However, ADCC levels in the PBNs and HANs of ARDS patients were much lower than in normal subjects (p < 0.01). The bacterial killing ability of PBNs was also impaired in ARDS patients, compared with either the normal control subjects or non-ARDS respiratory failure patients. In conclusion, chemiluminescence is increased in both PBNs and HANs of ARDS patients, which could partly explain the pathogenesis of acute lung injury. Furthermore, the suppression of ADCC in both PBNs and HANs as well as the impaired bacterial killing of PBNs may explain the impaired bacterial defenses and the consequent increase in pulmonary infection among ARDS patients.