A computerized bibliographic search of published research and a citation review of English-language publications about prone positioning of patients with acute respiratory distress syndrome were done. Information on prone positioning related to technique, patients' responses, complications, and recommendations to prevent complications was extracted. In the 20 pertinent clinical studies found, 297 patients (mean age, 39 years) with acute respiratory failure were positioned prone. Timing from the onset of respiratory failure to when the patient was first positioned prone varied, as did the frequency of prone positioning. Patients spent from 30 minutes to 42 hours prone. In 47% of the studies in which abdominal position was noted, chest and pelvic cushions were used to allow the abdomen to protrude while the patient was prone. Improved oxygenation within 2 hours was reported in 69% of patients, and the improvements were cumulative and persistent. Aside from early intervention, factors predictive of patients' responses were inconsistent, and patients' initial responses were not predictive of subsequent responses. Iatrogenic critical events were rare. Dependent edema of the face was prevalent. Pressure ulcers were reported in studies with longer periods of prone positioning. The most serious complication, corneal abrasion requiring corneal transplantation, was reported in one patient. Clinical knowledge about prone positioning is limited. Phase 1 studies focusing on how to safely turn and care for critically ill patients positioned prone for prolonged periods are needed.