BACKGROUND: Patients with heart disease often suffer from difficulties in psychological adaptation during cardiac rehabilitation. Mood disorders such as depression are known to be highly prevalent in cardiac patients and to have a negative impact on the progression of coronary heart disease. However, cardiac patients have difficulties to get psychological treatments due to low availability and motivational difficulties. Web-based interventions have been proven to be effective in treating depressive symptoms. Deprexis is a promising web-based psychological treatment which was devised for depressed patients. The aim of the study InterHerz is to examine if Deprexis is an effective psychological treatment to reduce stress and depression in cardiac patients. METHODS/DESIGN: The sample will consist of 80 depressed patients randomized to an intervention group or a waitlist (10 weeks). Patients are recruited via cardiologists, cardiac rehabilitation units and the website of the Swiss Heart Foundation. Patients have access to a guided self-help program in which they work themselves through several modules and receive feedback from a clinical psychologist. Pre- and post-assessments, and a six-month follow-up, are conducted using online questionnaires and diagnostic interviews. DISCUSSION: Deprexis is a new web-based treatment which has the potential to help depressed cardiac patients with limited access to psychological treatment to increase their mental health. TRIAL REGISTRATION: Current Controlled Trials ISRCTN45945396.