CLL (chronic lymphocytic leukemia) is the most common form of leukemia in Western population. The most frequent aberration in CLL is the deletion of 13q14 which involves the D13S319 locus and which is associated with a favorable prognosis if occurring as the sole genetic aberration. Deletions of the long arm of chromosome 13 are also frequently detected in patients with aggressive non-Hodgkin lymphoma (NHL) and have been found to represent an adverse prognostic factor in MM. Trisomy 12 represents another frequent chromosomal aberration in CLL, detected in about 20% of CLL cases. Trisomy 12 as single aberration is associated with an intermediate prognostic outcome. Hence, in combination with further biological markers, morphology and clinical information FISH is a valuable tool to predict disease progression and overall survival in CLL patients.