The c-myc proto-oncogene is activated by translocation in Burkitt's lymphoma and substitutions in codon 58 stabilize the Myc protein or augment its oncogenic potential. In wild-type Myc, phosphorylation of Ser 62 and Thr 58 provide a landing pad for the peptidyl prolyl-isomerase Pin1, which in turn promotes Ser 62 dephosphorylation and Myc degradation. However, the role of Pin1 in Myc-induced lymphomagenesis remains unknown. We show here that genetic ablation of Pin1 reduces lymphomagenesis in Eµ-myc transgenic mice. In both Pin1-deficient B-cells and MEFs, the proliferative response to Myc was selectively impaired, with no alterations in Myc-induced apoptosis or mitogen-induced cell cycle entry. This proliferative defect wasn't attributable to alterations in either Ser 62 phosphorylation or Myc-regulated transcription, but to the indirect activation of an Arf-p53 dependent cytostatic response. Pin1 silencing in lymphomas retarded disease progression in mice, making Pin1 an attractive therapeutic target in Myc-driven tumors. RNAseq samples of Pin1+/+ control (n=4), Pin1 -/- control (n=2), Pin1+/+ Eµ-myc pre-tumoral (n=3), and Pin1-/- Eµ-myc pre-tumoral (n=4) B cells.