This prospective, randomised study was conducted to assess the effect of flexible laryngeal mask airway (FLMA) size on oropharyngeal leak pressure (OLP) in children at the recommended intracuff pressure. A total of 120 children undergoing elective ophthalmic surgery were randomly assigned to the size 2 FLMA group or size 2.5 FLMA group. The primary measurement was OLP at an intracuff pressure of 40 cmH 2 O. Secondary outcomes included the incidence of OLP <10 cmH 2 O, insufficient ventilation, gastric insufflation, insertion time, successful first-attempt insertion rate, fibreoptic view grade and pharyngolaryngeal adverse events. The median OLP was comparable for the size 2 and size 2.5 FLMA (18 cmH 2 O versus 18 cmH 2 O, P=0.38). However, the size 2 FLMA group had a higher incidence of OLP <10 cmH 2 O and insufficient ventilation (13.3% versus 0, P=0.006). In subgroup analyses based on weight, the size 2.5 FLMA had a lower occurrence of OLP <10 cmH 2 O and insufficient ventilation (27% versus 0, P=0.0046) in children 16-20 kg. We conclude that at a 40 cmH 2 O intracuff pressure, the OLP with the size 2 and size 2.5 FLMA was similar in children weighing 10-15.9 kg. However, in children weighing 16-20 kg, size 2 devices had a higher incidence of low OLP and insufficient ventilation, so a 2.5 FLMA may be preferable in this subgroup.