Clinical outcome of mantle cell lymphoma (MCL) is highly heterogeneous. Tumor cell proliferation as assessed by the Ki-67 index has been shown to yield prognostic information on MCL in many studies using heterogeneously treated patient cohorts. The prognostic value of the Ki-67 index in patients treated with anti-CD20 therapy has not been studied so far. We analyzed the Ki-67 index at primary diagnosis in 249 advanced-stage MCL patients treated within randomized trials. Ki-67 showed high prognostic relevance for overall survival (relative risk 1.27 for 10% higher Ki-67, P < .001), also independently from clinical prognostic factors. The 3 groups with different Ki-67 index of less than 10%, 10% to less than 30%, and 30% or more showed significantly different overall survival in patients treated with CHOP (P = .001) as well as in patients treated with CHOP in combination with anti-CD20 therapy (R-CHOP, P = .013). Thus, the Ki-67 index remains an important prognostic marker in the era of anti-CD20 therapy. The Euro-pean MCL study is registered at www.ClinicalTrials.gov as #NCT00016887.