In our study, we aimed to investigate the role of angiotensin-converting enzyme 2 and angiotensin II plasma levels on prognosis and mortality in patients with isolated hypertension, patients with chronic diseases in addition to hypertension and patients with COVID-19 without comorbidities, in accordance with the use of renin–angiotensin–aldosterone system inhibitor. Here, ACE2 is linked to COVID-19.