The transition from primary to secondary school challenges children's psychological well-being. A cross-transitional longitudinal study (N = 306; mean age = 12.2 years) examined why some children's self-esteem decreases across the transition whereas other children's self-esteem does not. Children's expected social acceptance in secondary school was measured before the transition; their actually perceived social acceptance was measured after the transition. Self-esteem and Big Five personality traits were measured both pre- and posttransition. Self-esteem changed as a function of the discrepancy between children's expected and actually perceived social acceptance. Furthermore, neuroticism magnified self-esteem decreases when children's ‘hopes were dashed'—when they experienced disappointing levels of social acceptance. These findings provide longitudinal support for sociometer theory across the critical transition to secondary school.