Prognostic value of fluorescence in situ hybridization (FISH) in patients with chronic lymphocytic leukemia Objectives: The aim of study is to test prevalence of cytogenetic changes in patients with chronic lymphocytic leukemia and to examine whether patients with proven cytogenetic changes have a worse therapeutic response to conventional chemoimmunotherapy and shorter survival compared to patients without proven cytogenetic changes. Study design: cross-sectional survey with historical data. Participants and methods: Subjects of the study are patients with chronic lymphocytic leukemia who were diagnosed with FISH analysis and whose disease was diagnosed between January 1, 2017 and January 1, 2021 at the Osijek Clinical Hospital Center. Results: The research was conducted on 62 patients, of whom 37 (60 %) were men and 25 (40 %) were women. The median age of the patients is 67 years, ranging from a minimum of 33 to a maximum of 83 years. Cytogenetic changes were proven by the FISH method in 31 (50 %) patients, most often trisomy 12 in 9 (15 %) patients and del 13q14.3 in 8 (13 %) patients. Stable disease was achieved in 4/28 (14 %) patients, partial remission was achieved in 17/28 (61 %) patients, and complete remission was achieved in 7/28 (25 %) patients. 7/62 (11 %) patients had a fatal outcome. Overall survival (OS) with regard to cytogenetic changes is statistically significantly shorter in patients with proven cytogenetic changes (log-rank test, P = 0.03). Conclusion: No significant difference was found in the response to therapy and the outcome of the disease, but overall survival in patients with proven cytogenetic changes is shorter compared to patients without proven cytogenetic changes.