Background The complications from coronavirus disease 2019 (COVID-19) have been the subject of study in diverse scientific reports. However, many aspects that influence the prognosis of the disease are still unknown, such as frailty, which inherently reduces resistance to disease and makes people more vulnerable. This study aimed to explore the complications of COVID-19 in patients admitted to a third-level hospital and to evaluate the relationship between these complications and frailty. Methods An observational, descriptive, prospective study was performed in 2020. A sample of 254 patients from a database of 3,112 patients admitted to a high-level hospital in Madrid, Spain was analyzed. To assess frailty (independent variable) the Clinical Frailty Scale (CFS) was used. The outcome variables were sociodemographic and clinical, which included complications, length of stay, intensive care unit (ICU) admission and prognosis. Results A total of 13.39% of the patients were pre-frail and 17.32% were frail. Frail individuals had a shorter hospital stay, less ICU admission, higher mortality and delirium, with statistical significance. Conclusion Frailty assessment is a crucial approach in patients with COVID-19, given a higher mortality rate has been demonstrated amongst frail patients. The CFS could be a predictor of mortality in COVID-19.