The BiDirect study examines the association between depression and cardiovascular diseases in an innovative design. The study has been funded by the German Minister of Research and Education since 2009. Patients, hospitalised for an acute depression episode, a random sample of the inhabitants of the city of Münster and patients with an acute coronary syndrome from the same area are recruited into three cohorts, examined in parallel with identical methods. The latter includes a psychiatric classification, including subtypes of depression, a vascular diagnostic work-up, weight and height assessments and magnetic resonance imaging (MRI) of the brain. A first follow-up will be conducted 2 years after baseline. The study design allows a comprehensive analysis of the complex and bidirectional association between subclinical arteriosclerosis and depression.