Background: The Jak2 (Janus kinase 2) / Stat5a (signal transduction and activators of transcription 5a) pathway plays a key role in regulating cell survival, proliferation and immune function. JAK2 is a non-receptor tyrosine kinase which is stimulated by a variety of cytokines. Moreover the JAK2 / STAT5 axis is activated by hypoxia and involved in regulating angiogenesis. We therefore hypothesize that variations in the JAK2 gene may predict outcome in patients with metastatic colorectal cancer (mCRC) treated with first-line FOLFIRI and bevacizumab (bev). Methods: Theimpact of 2 functional SNPs within the JAK2 and STAT5a genes on outcome was evaluated in 295 pts with mCRC treated with first-line FOLFIRI / bev in the randomized phase III FIRE-3 trial. 227 pts receiving FOLFIRI and cetuximab (FIRE-3) served as a negative control. Genomic DNA was extracted from formalin fixed paraffin embedded tissue and the SNPs were analyzed by PCR-based direct sequencing. Results: Baseline characteristics in the FOLFIRI / bev arm were as follows: female:male ratio = 1/3:2/3; median age = 65y (31-76) andmedian PFS/OS = 10.1/24.2 months. The JAK2 rs2274472 SNP showed significant association with progression-free survival (PFS). C allele carriers had a shorter median PFS compared to those with a T/T genotype (9.9 vs 11.7 months) in both univariate (HR 1.30, 95% CI 1.00-1.69, p = 0.045) and multivariate analysis (HR 1.39, 95% CI 1.06-1.83, p = 0.018). However, this association could not be observed in patients treated with FOLFIRI and cetuximab. Here, patients harboring any C allele and T/T genotype carriers showed an equal median PFS (10.0 vs 9.9 months, HR 1.00, 95% CI 0.75-1.33, p = 1.00). Conclusions: Our results provide the first evidence that the JAK2 polymorphism rs2274472 might serve as a predictive marker in pts with mCRC treated with FOLFIRI and bev in the first line setting. Targeting the JAK2 / STAT5A axis might be a promising approach to further enlarge our treatment options against mCRC and to overcome resistance to anti-angiogenic therapy.