Background: The novel coronavirus pandemic, severe acute respiratory syndrome CoV-2 (SARS COV-2), has become a global threat and rapidly spread worldwide. The COVID-19 pandemic has posed a number of challenges, the most notable of which is the management of patients with chronic underlying diseases. Objectives: The present study aimed to evaluate the risk of COVID-19 severity and mortality in patients with chronic underlying diseases. Methods: In this retrospective cohort study, the data on the disease severity and morality of confirmed COVID-19 patients admitted to Baharloo Hospital, Tehran, Iran, from February 2020 to March 2020 were analyzed and reported. Patients' characteristics, including age, gender, and underlying diseases, were also considered. Results: The study encompassed 1244 patients with the mean age of 53.29 years, among whom there were 573 patients with at least one co-existing chronic disease. Hypertension, diabetes mellitus (DM), and ischemic heart disease (IHD) were the most common coexisting chronic diseases. The findings revealed that underlying diseases were significantly associated with disease mortality and severity. Conclusions: The findings showed that patients with comorbidities were significantly at higher risk of death and severe forms of COVID-19. In this regard, patients with underlying diseases should be of concern.