Background: The development of supraglottic airway devices has revolutionized airway management during general anaesthesia. Two devices are widely used in clinical practice to facilitate positive pressure ventilation: the ProSeal laryngeal mask airway (pLMA) and the Classic laryngeal mask airway (cLMA). It is not clear whether these devices have important clinical differences in terms of efficacy or complications. Objectives: To compare the effectiveness of the ProSeal laryngeal mask airway (pLMA) and the Classic LMA (cLMA) for positive pressure ventilation in adults undergoing elective surgery. Search methods: We searched the Cochrane Central Register of Controlled Trials (CENTRAL; 2017, Issue 3) in the Cochrane Library; MEDLINE (Ovid SP, 1997 to April 2017); Embase (Ovid SP, 1997 to April 2017); the Institute for Scientific Information (ISI) Web of Science (1946 to April 2017); and the Cumulative Index to Nursing and Allied Health Literature (CINAHL) (EBSCO host, 1982 to April 2017). We searched trial registries for ongoing studies to April 2017. We did not impose language restrictions. We restricted our search to the time from 1997 to April 2017 because pLMA was introduced into clinical practice in the year 2000. Selection criteria: We included randomized controlled trials (RCTs) that compared the effectiveness of pLMA and cLMA for positive pressure ventilation in adults undergoing elective surgery. We planned to include only data related to the first phase of cross-over RCTs. Data collection and analysis: We used standard methodological procedures expected by the Cochrane Collaboration. Main results: We included eight RCTs that involved a total of 829 participants (416 and 413 participants in the pLMA and cLMA groups, respectively). We identified six cross-over studies that are awaiting classification; one is completed but has not been published, and data related to the first treatment period for the other five studies were not yet available. Seven included studies provided data related to the primary outcome, and eight studies provided data related to more than one secondary outcome. Our analysis was hampered by the fact that a large proportion of the included studies reported no events in either study arm. No studies reported significant differences between devices in relation to the primary review outcome: failure to adequately mechanically ventilate. We evaluated this outcome by assessing two variables: inadequate oxygenation (risk ratio (RR) 0.75, 95% confidence interval (CI) 0.17 to 3.31; four studies, N = 617) and inadequate ventilation (not estimable; one study, N = 80). More time was required to establish an effective airway using pLMA (mean difference (MD) 10.12 seconds, 95% CI 5.04 to 15.21; P < 0.0001; I2 = 73%; two studies, N = 434). Peak airway pressure during positive pressure ventilation was lower in cLMA participants (MD 0.84, 95% CI 0.02 to 1.67; P = 0.04; I2 = 0%; four studies, N = 259). Mean oropharyngeal leak (OPL) pressure was higher in pLMA participants (MD 6.93, 95% CI 4.23 to 9.62; P < 0.00001; I2 = 87%; six studies, N = 709). The quality of evidence for all outcomes, as assessed by GRADE score, is low mainly owing to issues related to blinding and imprecision. Data show no important differences between devices with regard to failure to insert the device, use of an alternate device, mucosal injury, sore throat, bronchospasm, gastric insufflation, regurgitation, coughing, and excessive leak. Data were insufficient to allow estimation of differences for obstruction related to the device. None of the studies reported postoperative nausea and vomiting as an outcome. Authors' conclusions: We are uncertain about the effects of either of the airway devices in terms of failure of oxygenation or ventilation because there were very few events. Results were uncertain in terms of differences for several complications. Low-quality evidence suggests that the ProSeal laryngeal mask airway makes a better seal and therefore may be more suitable than the Classic laryngeal mask airway for positive pressure ventilation. The Classic laryngeal mask airway may be quicker to insert, but this is unlikely to be clinically meaningful.