Objective: To describe the concerns and modes of action of Finnish school nurses during pupils' health check-ups. Methods: Focus group interviews with 17 school nurses were performed in 2011 and again in 2013. Data were analysed using inductive content analysis. Results: School nurses' concerns were mostly associated with the psychosocial development of pupils and with problems in the family, for example, the loss of a parental role. With regard to physical health, school nurses were particularly concerned with obesity among pupils. The new phenomenon of cyberbullying also caused concern. School nurses were able to identify the causes for their concern at school health check-ups, but they were uncertain regarding how they should act and finding the right mode of action and sufficient multidisciplinary support when pupils had problems. Conclusion: The concern of school nurses at health check-ups was mainly associated with pupils' health habits, their psychosocial development and family characteristics, and with addressing the need to taking the health behaviours and well-being of the entire family into consideration during health check-ups. Linked-up processes in school health care and multidisciplinary collaboration within schools are of primary importance in supporting the work of nurses and in promoting pupils' well-being and health.