The adaptor molecule Lnk negatively regulates signaling downstream of cytokine receptors in self-renewal of hematopoietic stem cells, proliferation of B precursors and differentiation/maturation of megakaryocytes. The circulating number of platelets in Lnk-deficient mice is increased by 5-folds to control, presumably owing to its negative effects on TPO-elicited signaling. Lnk expression is maintained in platelets, however, the precise functions in platelets remain unknown. Here we report that Lnk deficiency results in the prolonged bleeding time (2-folds, p<0.01) due to, at least in part, defective the outside-in signaling from α IIbβ 3 integrin. Lnk-null platelets exhibited less activation of PAK kinase, a Rac effector in fibrinogen-adherent platelets. These were concomitant with the defect of lamellipodial formation upon adhesion to fibrinogen in the absence of Lnk, compared to control platelets (p<0.01). Defective Lnk-null platelet functions were also demonstrated by the reduced three-dimensional growth (height) of platelet thrombi on the collagen surface under whole blood flow condition with the adjusted same platelet number at high wall shear rate of 1,500 s-1 (8.1±1.4 μ m in Lnk-deficient vs. 12.1±2.2 μ m in control, p<0.001), while the two-dimensional surface coverage was not impaired. On the other hand, there was no difference between Lnk-null and control platelets in aggregation and α IIbβ 3 integrin activation in response to stimulation with thrombin or collagen. Taken together, implication of Lnk to sites of adhesion may explain the phenotype of Lnk-deficient platelet function. The findings indicate that Lnk is a unique “dual-functioning” scaffolding molecule, which upregulates thrombus formation, while suppressing cytokine-mediated signaling.