The lifetime risk of coronary artery disease (CAD) is doubled in people with a family history of premature disease, yet this risk is not captured in most 5- or 10-year risk assessment algorithms. Coronary artery calcium scoring (CCS) is a marker of subclinical CAD risk, which has been shown in observational studies to provide prognostic information that is incremental to clinical assessment; is relatively inexpensive; and is performed with a small radiation dose. However, the use of CCS in guiding prevention is not strongly supported by guidelines. Showing definitive evidence of the efficacy and cost-effectiveness of CCS is therefore of importance.The proposed randomized controlled trial of the use of CCS will be targeted to 40- to 70-year-old first-degree relatives of patients with CAD onset <60 years old or second-degree relatives of patients with onset <50 years old. Control patients will undergo standard risk scoring and be blinded to CCS results. In the intervention group, primary prevention in patients undergoing CCS will be informed by this score. At 3 years, effectiveness will be assessed on change in plaque volume at computed tomography coronary angiography, the extent of which has been strongly linked to outcome.The CAUGHT-CAD trial will provide evidence to inform the guidelines regarding the place of CCS in decision making regarding primary prevention of patients with a family history of premature disease.