Drop-out rates are high in many lifestyle programmes. To promote adherence, the aim of this study was to identify mediators of participation in a diet and exercise (DE) intervention in a general population.Data were baseline data from a randomized non-pharmacological clinical trial in Copenhagen during 1999-2001. The participation rate was 53.3%. Participants at high risk of ischaemic heart disease (IHD) and who were offered participation in a DE counselling group intervention were included (N = 2022). Clinical characteristics, and demographic, psychosocial and lifestyle factors were measured.Mediators of acceptance of participation were awareness of an unhealthy lifestyle or a bad health, low self-rated care of own health, perceived susceptibility of cardiovascular disease (CVD; overall and associated with lifestyle), high degree of motivation towards dietary changes and low self-efficacy about increasing physical activity. Overweight and impaired glucose tolerance (IGT)/screen-detected diabetes predicted acceptance whereas an absolute risk score for IHD was inverse associated with acceptance. Mediators of high adherence were low self-efficacy about changing dietary habits and perceived susceptibility of CVD and furthermore screen-detected diabetes and overweight predicted high adherence.Awareness of unhealthy lifestyle, perceived susceptibility of disease and motivation towards lifestyle changes were important mediators of participation. Screen-detected diabetes/IGT predicted participation and adherence whereas overweight individuals were more likely to accept but also to drop out of the course. The use of an absolute risk score in health promotion should be further evaluated.