This trial was designed to compare early surgery with watchful waiting for persistent bilateral otitis media with effusion (OME) in terms of hearing loss and behavioural problems in pre-school children. A randomised controlled trial compared the effects of early treatment with ventilation tubes versus watchful waiting for 9 months. Results were analysed by intention-to-treat. One hundred and eighty-two pre-school children (date of birth between 1 April 1991 and 31 December 1992), mean age of 2.9 years (SD 0.85) with at least a 3-month history of bilateral OME and hearing loss of greater than 25 dB were treated in Bristol Children's Hospital between November 1993 and January 1996. Bilateral ventilation tubes were inserted within 6 weeks of randomisation or within 6 weeks of reassessment after 9 months of watchful waiting, with a final assessment at 18 months. The main outcome measures were behavioural problems, measured by the Richman Behaviour Checklist, and hearing loss at 4000 Hz in the better hearing ear. Early surgical intervention significantly reduced behavioural problems by 17% (95% CI, 2% - 33%). This difference was largely mediated by concurrent hearing loss. After 18 months, there was no longer a significant difference (95% Cl, -19% to +10%). However, the majority (85%) of the Watchful Waiting group had required surgery and 22% of all children still had behavioural problems.