In chronic lymphocytic leukemia (CLL), chromosomes usually evade detailed cytogenetic analyses because cells poorly respond to the traditionally used set of mitogens. We applied novel technologies, such as stimulation of CLL cells either with CD40 ligand or with a combination of CpG-oligodeoxynucleotides and IL-2, to increase the frequency of metaphase spreads for detailed chromosome analysis in 96 patients with CLL. This approach revealed that translocations occurred in 33 of 96 (34%) of our patients with CLL. The presence of translocations defined a new prognostic subgroup because these patients have significantly shorter median treatment-free survival (24 months vs 106 months; P < .001) and significantly inferior overall survival (OS; median, 94 months) than patients without translocations (346 months; P < .001). In multivariate analysis-including Binet stage, complex karyotype, CD38 expression, and 17p deletions-translocation proved to be the prognostic marker with the highest impact for an unfavorable clinical outcome (P < .001). In summary, we identified a new subgroup of patients with CLL defined by chromosomal trans-locations and poor prognosis. Our data may facilitate the identification of molecular events crucial for transforming activity in this disease and should have implications for risk-adapted clinical management of patients with CLL.