Aims: To determine whether distinct trends can exist in children's diurnal cortisol slopes as they transition to school, and the extent to which these trends relate to preschool attendance and/or exerted effortful control. Method: A secondary analysis of the anonymised data gathered for the UK Transition to School Study was carried out. 105 children were studied over a 12-month period during transition to school at mean age 55 months. Children's diurnal cortisol slopes were measured as the difference between average salivary cortisol concentrations (SCC) sampled at waking and early evening over two days at each of three measurement time points: 4 months before, 2 weeks after, and 6 months after school entry. Children's effortful control was measured at 2 weeks after school entry using the parent-administered Child Behavior Questionnaire. Parental questionnaires recorded the duration children spent in preschool (months; days per week; hours per week), and four background characteristics: child gender, parental cohabitation, responding parent's age, and responding parent's level of education. Findings: Latent class growth analysis suggested two distinct trends in diurnal cortisol slopes during children's transition to school: 39% of children demonstrated flatter diurnal cortisol slopes. These children were likely to have spent fewer hours per week in preschool, and were likely to exert less effortful control 2 weeks after transitioning to school. These associations underscore the importance of continuity in children's daily routines as they transition to school. Implications are discussed concerning school readiness and the effectiveness of early interventions.