Child Health Services (CHS) form a strategic arena for Swedish health promotion, from a societal as well as individual perspective. The present study focuses on district nurses´ health counselling in CHS. The aim is to improve understanding of what is important in shaping and accomplishing health promotion, and to generate theory useful in a CHS context. Data were collected at CHS in primary health care. Data sources consist of video-observations with recalled interviews, field notes, notes from group discussions, individual interviews and data from my earlier study of health counselling. Data analysis followed the classic Grounded Theory (GT) method. Empirical data indicated that health promotion in CHS has two approaches, a population approach and an individual approach. District nurses thus have two missions: a public health mission (a government demand based on families as parts of the population), and a family health mission (a request from individual families). District nurses' main concern in CHS is to synthesise these two opposing missions and approaches in their practical work at the CHC (Child Health Centres). The category Population-individualisation evolved as the core category. Four dynamic fields between public and family health missions and four patterns for synthesis evolved. Through the generating analysis a substantive theory could be formulated. The generated theory grounded in empirical data within the context of CHS provides us with knowledge that can help district nurses understand and manage their main concern for synthesising these two missions. This theory may also contribute to increased awareness among administrators, decision makers and nurse- and health promotion educators with regard to the double mission in health promotion and the strategies for coping with it. In this way, generated theory should be able to contribute to improve health promotion and health counselling in CHS.