The preschool years are when the foundations for subsequent development are laid. Otitis media is a prevalent early life health issue. Understanding its impact on children’s development is crucial. We will determine whether the number of ear infections in the preschool years is associated with an increased risk of poor hearing and of poorer language, cognitive, social or behavioural development at age 4½ years. We will investigate what impact the medical and surgical management of otitis media has on these outcomes. We will complete this project within an established ethnically and socioeconomically diverse cohort of New Zealand children. We will collect objective measures of otitis media episodes and of hearing, language, cognitive, social or behavioural development. Hearing impairment during childhood has long term effects on language, communication, education and behaviour. We will determine the potential for these outcomes to be improved by the prevention and timely treatment of otitis media.