Smartphones are becoming widely popular and the number of users is significantly increasing, reaching over 65% in South Korea in 2013 and the children begin to use a smartphone at earlier age. Earlier and higher exposure of multimedia is known to have negative effects on children's physical and mental status.

The aim of the present study was to examine young children's exposure to smartphone and identify the effects of high exposure of smartphone on children's behaviors among Korean children from ages 3–5 years.

In 2014–2015, the parents of 400 children aged 3–5 years (207 boys and 193 girls) were surveyed using a questionnaire on the use of smartphone, children's behaviors, temperaments, social and language development at 3 community-based children's mental health centers.

Many children used televisions (95.5%), computers (37.3%) or tablet PC (36.2%), and smartphones (84.6%). Most (74.2%) started using mobile medias before age 2. Parents gave children devices like smartphones to keep them calm (60.8%), when being busy doing something (52.2%), and at playtime (34.3%). The children's age at first smartphone use and the frequency were not associated with children's behaviors and temperaments. Higher use group (> 2 h/d) show more somatic symptoms (OR 8.97, P < .001), more attention problem(OR 4.43, P < .001), more aggressive symptoms (OR 1.30, P < .001) and more withdrawal symptoms(OR 1.22, P < .001) than lower use group.

Young children in Korean urban communities had almost universal exposure to mobile devices, especially smartphone. Early and severe exposure of smartphone by young children aged 3–5 years is highly associated with children's behaviour problems like both internalising and externalising problems.

The authors have not supplied their declaration of competing interest.