BACKGROUND: Patients with acute respiratory distress syndrome (ARDS) and acute lung injury (ALI) require mechanical ventilatory support. ALI/ARDS is further complicated by ventilator-induced lung injury. Lung-protective ventilation strategies may lead to improved survival. OBJECTIVES: To assess the effects of ventilation with lower tidal volume (Vt) on morbidity and mortality in adults patients affected by ALI/ARDS. A secondary objective was to determine whether the comparison between low and conventional Vt is different if a plateau airway pressure of greater than 30 to 35 cm H20 was used. SEARCH STRATEGY: We searched The Cochrane Central Register of Controlled Trials (CENTRAL), The Cochrane Library issue 3, 2002; MEDLINE (1966 to June 2002); EMBASE and CINAHL (1982 to June 2002); intensive care journals and conference proceedings; databases of ongoing research, reference lists and 'grey literature'. SELECTION CRITERIA: Randomized trials comparing ventilation using lower Vt and/or low airway driving pressure (plateau pressure 30 cm H2O or less), resulting in Vt of 7 ml/kg or less versus ventilation that uses Vt in the range of 10 to 15 ml/kg, in adults (16 year-old or greater). DATA COLLECTION AND ANALYSIS: Two reviewers independently assessed trial quality and extracted data. Wherever appropriate, results were pooled. Fixed and random effects models were applied. MAIN RESULTS: Five trials, involving 1202 patients, were eligible. The test for heterogeneity gave a P-value of 0.12. Ventilation with lower Vt was associated with a decreased mortality at the end of the follow up period for each trial: 216/605 (35.7%) versus 249/597 (41.7%), relative risk (RR) 0.85 (CI 0.74 to 0.98). The effect of the intervention was not statistically significant when a random effects model was used: RR 0.91 (CI 0.72 to 1.14). Mortality at day 28 was significantly reduced by lung-protective ventilation: RR 0.74 (CI 0.61 to 0.88). The comparison between low and conventional Vt was not significantly different if a plateau pressure less than or equal to 31 cm H2O in control group was used: RR 1.13 (CI 0.88 to 1.45). There was insufficient evidence about morbidity and long term outcomes. REVIEWER'S CONCLUSIONS: Clinical heterogeneity, such as different lengths of follow up and higher plateau pressure in control arms in two trials make the interpretation of the combined results difficult. Mortality is significantly reduced at day 28 and the effects on long term mortality are uncertain, although the possibility of a clinically relevant benefit cannot be excluded. There is no evidence that low Vt ventilation is beneficial in patients where hypercapnia is potentially harmful.