Alveolar recruitment is one of the beneficial effects of prone positioning in patients with ARDS (acute respiratory distress syndrome). However, responses vary among patients and, therefore, we hypothesized that alveolar recruitment is an individual time-dependent process and its measurement might be helpful to 'dose' prone positioning individually. In 13 patients diagnosed with ARDS, EELV (end-expiratory lung volume) was measured in the supine position, immediately after turning to the prone position, at 1, 2, 4 and 8 h in the prone position and after returning to the supine position. Responders were defined based on a 30% increase in oxygenation. EELV increased in responders, whereas it remained constant in non-responders. The time course was different in individual patients. In some responders, a plateau was reached as early as 2-4 h, whereas, in others, 8 h of prone positioning was not sufficient to allow complete recruitment. The increase in lung volume was associated with both an increase in arterial oxygenation and a decrease in venous admixture. Furthermore, responders had significantly lower baseline EELVs than non-responders. In conclusion, alveolar recruitment during prone positioning has been characterized as an individual time-dependent process. Its measurement might be useful to apply prone positioning more individually and might also help to identify responders.