Background Health Trainers were introduced in the 2004 ‘Choosing Health’ White Paper with the aim of promoting healthy lifestyle behaviours among people living in disadvantaged areas. This is achieved through the use of techniques based on evidence and theories from health psychology. The initiative also aims to combat social exclusion by recruiting and training local people as Health Trainers. The evidence to support the effectiveness of behaviour change interventions delivered by non-professional or lay workers is mixed. Furthermore, there is a tendency to emphasise individual behaviour change outcomes rather than assessing the effects of community-based programmes across the breadth of their activities. Qualitative research can help to shed light on these effects and inform the successful replication and sustainability of interventions. Objectives To explore staff and service user views, experiences and perceptions of the Health Trainers intervention in order to evaluate its perceived effects on individuals, organisations and communities. Design A longitudinal qualitative research approach was used to provide insight into the participants' experiences and social contexts. In-depth interviews with service users were conducted at 0, 3, 6 and 12 months, and with staff at 0 and 12 months. The interviews were audio-recorded, transcribed verbatim and analysed using the constant comparative method associated with grounded theory. Setting and Participants 26 service users and 18 staff members theoretically sampled from 3 Health Trainer Services in the north east of England. Findings Analyses of the interview data identified an array of short-and medium- effects perceived as arising from participation in the Health Trainers intervention. These have been grouped into four categories using Nutbeam's outcome model for health promotion. Health promotion actions: Community members were successfully recruited as Health Trainers and perceived to be delivering interventions that are inclusive and acceptable to local people. Health promotion outcomes: Service users reported changes in knowledge, attitudes and personal skills relating to key lifestyle issues, as well as increased self-efficacy. Intermediate health outcomes: In the majority of cases, health-related behaviour changes made by 3 months were reportedly sustained at 6 and 12 months. Health and social outcomes: Longer-term effects included improvements in existing conditions, reduced medication usage and enhanced quality of life. Conclusion This study adds to the evidence base for the evaluation of community-based interventions designed to improve health and reduce health inequalities. Use of a longitudinal qualitative approach was successful in capturing the complexity of the Health Trainers intervention and identifying its short- and medium-term effects.