To examine the potential impact of visual personalized biomarker feedback on intention to stop smoking and to evaluate possible underlying causal pathways. This study is a pilot for a randomized controlled trial. Outcome measures were assessed immediately after the intervention and at 4 weeks follow-up. Twenty-three smokers attending a cardiovascular outpatient clinic in London were randomly allocated to one of two groups: to either receive a print-out of an ultrasound image of their carotid artery showing atherosclerotic plaque alongside an image of a disease-free artery, or to receive routine verbal feedback. The intervention significantly increased perceptions of susceptibility to smoking-related diseases (Cohen's h=0.99) and led to increases both in engagement in smoking cessation behaviours (Cohen's h=0.79) and intentions to stop smoking (Cohn's d=0.44). The latter was moderated by self-efficacy: the intervention increased intention to stop smoking only in people with higher levels of self-efficacy with regard to stopping smoking. This study provides preliminary support for the potential effectiveness of personalized biomarker feedback to increase intentions to stop smoking. It also highlights the need to target and increase self-efficacy in smoking cessation interventions.