Background: Evidence from Western countries indicates marked increases in diagnosis and treatment of childhood psychiatric disorders in recent years. These could reflect changes in prevalence of mental health problems, changes in their impact or increased clinical recognition and help-seeking. Epidemiological cross-cohort comparisons are required to test possible changes in prevalence, but are lacking for pre-adolescent children in Great Britain. Methods: Parent and teacher Strength and Difficulties Questionnaire (SDQ) ratings were used to compare rates of emotional, conduct and hyperactivity problems in 7-year-old children across three nationally representative British samples assessed in 1999 (n = 1033), 2004 (n = 648) and 2008 (n = 13 857). The SDQ impact supplement was used to assess associated distress, social, and educational impairment. Stratified analyses examined trends by gender and socio-economic group. Results: There was a decline in mean problem scores and a fall in the percentages scoring in the "abnormal" range for all symptom types across the period of study. This decline was observed for all demographic groups, for parent and teacher reports, and was more marked for boys than girls. Both parent- and teacher-rated impact scores differed across the three cohorts for boys. Teacher-rated impact scores differed across cohorts for girls. Conclusions: The first decade of the 21st Century saw a reduction in perceived levels of emotional and behaviour problems in pre-adolescent children in Great Britain. The threshold at which mental health problems have an impact on children's distress and classroom learning has changed over time. Continued monitoring of child mental health remains a priority.