Background: Chronic lymphocytic leukemia (CLL) is the most frequent B-cell lymphoproliferative disorder in adults. Although its cytogenetic landscape is dominated by 13q deletion, trisomy 12, 11q deletion, and 17p deletion, rarer numerical abnormalities such as trisomy 3 are occasionally identified. The clinical impact and prognostic value of trisomy 3 remain poorly characterized. Case Presentation: A 71-year-old male presented with progressive asthenia and intermittent headaches lasting for several weeks. Physical examination revealed a tumor syndrome combining splenomegaly and peripheral lymphadenopathy. Laboratory evaluation demonstrated major hyperleukocytosis at 180000/µL associated with anemia (hemoglobin 8.0g/dL) and thrombocytopenia (platelets 75000/µL), categorizing the disease as Binet stage C (Rai stage IV). Peripheral blood smear showed a lymphocytosis composed of monotonous mature small lymphocytes with the presence of Gumprecht shadows. Flow cytometry immunophenotyping on whole blood confirmed the diagnosis of classic CLL with a Matutes score of 5. Conventional R-banding karyotype revealed a trisomy of chromosome 3. FISH analysis excluded the presence of targeted deletions of the TP53 (17p13) and ATM (11q22.3) genes. Conclusion: This case illustrates an aggressive initial presentation of CLL associated with an isolated trisomy 3, despite an immunophenotypic profile highly concordant with typical CLL. Systematic investigation and characterization of these rare chromosomal aberrations are essential to better define their pathophysiological role.