Adult T-cell leukemia/lymphoma is a distinct clinical entity characterized by a clonal proliferation of malignant T-lymphocytes. The etiologic agent of the disease is a Human T-cell lymphotropic virus type I. It occurs almost exclusively in areas where the virus is endemic; however the disease develops only in the minority of patients who are virus carriers. Karyotyping findings and their correlation with clinical features are still limited in T-cell malignancies, complicated by clinical heterogeneity and a plethora of secondary abnormalities. This study describes detailed chromosomal and fluorescence in situ hybridization results observed in a patient with adult T-cell leukemia/lymphoma and correlates them with clinical characteristics.