Giant cell myositis (GCMm) and giant cell myocarditis (GCMc) are two rare autoimmune conditions. Among these, GCMc is a life-threatening disease with a 1-year mortality rate of 70%. Lethal ventricular arrhythmias, rapid evolution to heart failure and sudden death risk makes GCMc an emergency condition. It is thought to be mediated by T-cells and characterized by the presence of myofiber necrosis and giant cells in biopsies. Most commonly co-manifesting conditions with GCMm and/or GCMc are thymoma, myasthenia gravis and orbital myositis, all of which are treatable. As suspicion is the key approach in diagnosis, the physician following patients with thymoma with or without myasthenia gravis and with orbital myositis should always be alert. The fatal nature of GCMc associated with these relatively benign diseases deserves a special emergency attention with prompt institution of combined immunosuppressive treatment and very early inclusion of heart failure teams.