Rationale, aims and objectives Cardiovascular disease (CVD) often arises from modifiable lifestyle factors. Health care professionals may lack the skills and resources to sustain behaviour change, lay ‘health trainers’ (LHT) offer a potential alternative. We sought to assess the cost-effectiveness of using a LHT to improve heart-health lifestyles in deprived communities. Methods Participants in this randomized trial were aged =18 years with at least one risk factor for CVD (hypertension, raised cholesterol, diabetes, BMI>30 or current smoker). Both groups received health promotion literature. LHT were also able to provide intervention participants with information, advice and support aimed at changing beliefs and behaviour. Costs and quality-adjusted life year (QALY) changes were estimated over 6 months. The cost-utility [incremental cost-effectiveness ratio (ICER)] of LHT was calculated and assessed in relation to the cost-effectiveness threshold of £20 000–30 000 per QALY. The probability of LHT being cost-effective was also calculated. Results Seventy-two participants were randomized to a LHT, with 38 controls. The mean cost of the LHT intervention was £151. On average, other health and social service costs fell by £21 for controls and £75 for intervention participants giving a LHT mean overall incremental cost of £98. The mean QALY gains were 0.022 and 0.028, respectively. The ICER for LHT was £14 480, yet there was a 61% chance of making the wrong decision at a £20 000/QALY threshold. Conclusion LHT provision was estimated to be cost-effective for people at risk of CVD. However, a large level of uncertainty was associated with that decision.