Marryat L, et al. - This population-based cohort study was performed to investigate socioeconomic inequalities in mental health at school entry and explore changes in these inequalities over the first 3 years of school. To help children from disadvantaged backgrounds, additional support at preschool and in early primary school could help in narrowing inequalities. Compared with their more advantaged peers, children from disadvantaged backgrounds started school with a higher prevalence of mental health difficulties, and this disparity widened markedly over the first 3 years of school.