Objectives: The main objective of this study is to investigate the incidence of lymphoma in patients with rheumatoid arthritis, with particular attention to demographic characteristics, age groups and gender differences. Study design: Historical cohort study. Respondents and methods: The study includes patients diagnosed with rheumatoid arthritis and hospitalized at the Department of Clinical Immunology and Rheumatology at KBC Zagreb from January 2021 to December 2023. The data, analyzed using SPSS software, were collected from the Hospital Information System (HIS) of KBC Zagreb and they include demographic characteristics, diagnoses, length of hospital stay and evolution of lymphoma. The Ethics Committee of KBC Zagreb approved the study and ensured anonymity and confidentiality of all collected data in accordance with the ethical principles and the Declaration of Helsinki. Results: This study analyzes data from patients with rheumatoid arthritis hospitalized at KBC Zagreb, including demographic characteristics, length of hospital stay and incidence of malignant neoplasms. The majority of mostly female patients are between 51 and 70 years old. Seropositive rheumatoid arthritis (M05) was present in 59.3 % of patients, while 40.7 % of patients had other forms (M06). The statistical analyzes revealed no significant differences in diagnoses and length of hospitalization between age groups and genders. Conclusion: The study found that the incidence of lymphoma in patients with rheumatoid arthritis is relatively low (2.5 %), but significant due to the severity of the disease. Although the differences in lymphoma incidence between different age groups and genders were not statistically significant, older patients tend to have longer hospital stays. Continuous monitoring and adaptation of therapeutic strategies are crucial for prevention and early detection of malignant complications in patients with rheumatoid arthritis.