We studied the size 4 laryngeal mask airway (LMA) to test the hypothesis that oropharyngeal leak pressure and fibreoptic position improves with increasing cuff volume. After LMA insertion, 50 anaesthetized adult patients had the cuff inflated in 5-ml increments to 40 ml. Oropharyngeal leak pressure was optimal at 15 ml and decreased at higher volumes. The fibreoptic position was optimal at 0-20 ml and deteriorated at higher volumes. Gastric insufflation was detected more frequently when the cuff volume exceeded 20 ml. We conclude that inflation of the size 4 LMA to the maximum recommended volume provides suboptimal conditions and that this value should be reduced from 30 to 20 ml.