A need for research on young children’s mental health has been identified. Moreover, there is a need to enhance teachers’ skills regarding health promotion work in preschool. The aim of the study is to examine the impact of a specific course on mental health promotion among Swedish preschool teachers. Data were gathered through interviews and documentation. A qualitative content analysis was used. Three categories: ‘attention’, ‘belonging’ and ‘personal growth’ – and six subcategories emerged from the analysis. The latent content of these categories is described in the theme: ‘Increased awareness, in thoughts and actions, of promoting mental health may create a sense of trust at preschools’. The results show that a specific course to promote mental health enhanced the teachers’ awareness of mental health which improved their work with children. This contributed to the development of a sense of trust at preschools, which is important to children’s mental health.