Randomized cross-over study of 19 patients on mechanical ventilation. Tc99m sulphur colloid labeling of stomach contents was performed, followed by sequential radioactivity counts in endobronchial secretions over a 5h period. Patients were randomized to the prone position or semi-recumbency and studied 12h later; the study was repeated 48h later with the patient in the alternate position. Radioactive counts were higher, indicating aspiration of gastric contents, in the prone position, and increased over time.