BACKGROUND: This randomized cross over study was performed to compare the effect of positive pressure ventilation (PPV) through a laryngeal mask airway (LMA) with that through an uncuffed endotracheal tube (ETT) in paralyzed pediatric patients. Additionally, this study was to determine the protective effect of LMA for the aspiration of oral secretion. METHODS: Pediatric patients (n = 158) weighing less than 30 kg, of ASA physical status 1 or 2 were studied. LMA or ETT was randomly selected and connected to a volume-type ventilator. After measurement and extubation, the other that was not initially used was inserted. Expiratory tidal volume (VT) and airway pressure were measured at the same ventilator setting by a simple pneumo-tachometer, and the expiratory tidal volume ratio (VTR) was calculated as expiratory VT of LMA/expiratory VT of ETT. In randomly selected 39 patients out of 158, methylene blue diluted 1:10 with saline was injected into the oral cavity during PPV through an LMA. At the end of the surgery, fiberoptic bronchoscopy was performed to observe any staining of methylene blue in the trachea. RESULTS: Successful placement of the LMA was achieved in 98% (155/158) of the cases. Expiratory VT through the LMA 1 or 1.5 was decreased if compared with that through the ETT. Leaking pressure of the LMA was lower than that of the ETT except LMA 2.5. VTR of the LMA size 1, 1.5, 2 and 2.5 (mean +/- SD) was respectively 0.94 +/- 0.22, 0.94 +/- 0.14, 1.02 +/- 0.18 and 1.06 +/- 0.19. There was no patient whose trachea was soiled with methylene blue. CONCLUSIONS: Only in older children who are indicated for LMA 2.5, did LMA have the same leakage and the same leaking pressure as ETT during PPV. LMA may not be recommended for the maintenance of PPV in young children less than 10 kg. Inflated LMA seems to have a protective effect against oral secretion.