Background: Clinical investigations of electric toothbrushes in young children are limited. Aim: To assess plaque reduction efficacy of an oscillating-rotating electric versus manual toothbrush in a paediatric population in primary and mixed dentitions. Design: In this randomised, single-brushing, 2-treatment, 4-period, replicate-use crossover study, subjects were divided into 2 age groups (3-6 years; 7-9 years) and assigned to a treatment sequence involving an Oral-B Kids electric brush and a manual brush control. Plaque was assessed pre- and post-brushing (Turesky Modified Quigley-Hein Plaque Index). Parents brushed the teeth of their children aged 3-6 years, whereas children aged 7-9 years brushed their own teeth under supervision. Plaque removal scores were analysed for brush differences in each age group separately using an analysis of covariance for crossover design. Results: Forty-one children (n = 20, 3-6 years; n = 21, 7-9 years) completed the study. For the primary dentition in children 3-6 years, the electric brush reduced 32.3% more plaque than the manual brush (P =.005). For the mixed dentition in children 7-9 years, the electric brush reduced 51.9% more plaque than the manual brush (P <.001). Conclusions: An electric toothbrush reduced significantly more plaque than a manual toothbrush in 2 paediatric age groups.