We have compared ease of insertion, oropharyngeal leak pressure, directly measured pharyngeal mucosal pressure and anatomical position (assessed fibreoptically) for the size 4 and size 5 laryngeal mask airway (LMA) in 20 male and 20 female patients. Microchip pressure sensors were attached to the LMA at locations corresponding to the piriform fossa, hypopharynx, base of the tongue, lateral and posterior pharynx, and the oropharynx. Oropharyngeal leak pressure, mucosal pressure and fibreoptic position were recorded during inflation of the cuff from 0 to 30 ml in 10-ml increments. In males, oropharyngeal leak pressure over the inflation range was higher for size 5 (21 vs 17 cm H2O; P = 0.01); mucosal pressure over the inflation range was higher in the posterior pharynx for size 4 (7 vs 2 cm H2O; P = 0.007), and higher in the piriform fossa (8 vs 5 cm H2O; P = 0.003) and hypopharynx (9 vs 5 cm H2O; P = 0.003) for size 5. In females, oropharyngeal leak pressure over the inflation range was the same (21 vs 21 cm H2O), but mucosal pressure over the inflation range was higher in the piriform fossa (21 vs 8 cm H2O; P = 0.003) and posterior pharynx (4 vs 2 cm H2O; P = 0.004) for size 4, and higher in the lateral pharynx (5 vs 1 cm H2O; P = 0.01) and oropharynx (11 vs 5 cm H2O; P = 0.009) for size 5. The distribution of mucosal pressure was different for size 4 between males and females, but not for size 5. For both males and females, fibreoptic position was similar. We conclude that the size 5 LMA is optimal in males, but either size is suitable for females. The shape of the pharynx may be different between males and females.