The correct diagnosis of tachycardia is important for reasons of treatment and prognosis. Intravenous bolus injection of adenosine can be used to diagnose tachycardia as ventricular or supraventricular and to convert AV-nodal re-entry tachycardia as well as re-entry tachycardia due to an accessory pathway. The effect of adenosine on different types of tachycardia is demonstrated. A strategy is proposed for examination and treatment of patients with manifest tachycardia to improve diagnosis and remove bias to supraventricular tachycardia and underdiagnosis of ventricular tachycardia.