The Most Common Infections and Their Treatments in Oncology Patients at Uppsala University Hospital Year 2020-2022 - a Retrospective Registry Study  Background: There are two types of infections of interest to this study, these are healthcareassociated infections and community-acquired infections. It is common for oncology patients to suffer from various difficult-to-treat infections caused by bacteria, viruses, and fungi. Death due to infections is the second most common cause in oncology patients. Antibiotics are a drug class of the type of antibacterial drugs that are widely used today.   Aim: The purpose of this study is to describe the frequency of an antibiotic use with a focus on the substance selection and indications as well as the resistance patterns in oncology patients at Akademiska hospital between the years 2020 and 2022.   Methods: The study was designed as a descriptive registry study, where different data sources were used as a method. Methods used to analyze the data were quantitative methods, providing a quantifiable answer to the sub-questions using numbers and structured data.    Results: The study indicated that the most common infections in oncology patients during these years were other community-acquired infection, urinary tract infection without fever, other skin/soft tissue/skeletal infections, and pneumonia. The most common treatment differed depending on which database the information was taken from. According to the first database, the most common preparations were Piperacillin/Tazobactam, Ciprofloxacin and Pivmecillinam. According to the other database, the most common preparations were Sulfamethoxazole/Trimethoprim, Ciprofloxacin and Piperacillin/Tazobactam. Of oncology patients treated with cancer drugs, 53% needed antibiotic treatment in hospital and 93% either administered in hospital or prescribed at home.   Conclusions: Many oncology patients suffer from infections that require treatment. This entails a high risk of antibiotic resistance. More study is needed for this patient group to optimize treatment and reduce relapse.