T umor-infiltrating blood vessels deviate morphologically and biochemically from normal vessels, raising the prospect of selective pharmacological targeting. Current antiangiogenic approaches focus mainly on endothelial cells, but recent data imply that targeting pericytes may provide additional benefits. Further development of these concepts will require deeper insight into mechanisms of pericyte recruitment and function in tumors. Here, we applied genetic tools to decipher the function of PDGF-B and PDGF-Rβ in pericyte recruitment in a mouse fibrosarcoma model. In tumors transplanted into PDGF-B retention motif–deficient (pdgf-b^ret/ret) mice, pericytes were fewer and were partially detached from the vessel wall, coinciding with increased tumor vessel diameter and hemorrhaging. Transgenic PDGF-B expression in tumor cells was able to increase the pericyte density in both WT and pdgf-b^ret/ret mice but failed to correct the pericyte detachment in pdgf-b^ret/ret mice. Coinjection of exogenous pericytes and tumor cells showed that pericytes require PDGF-Rβ for recruitment to tumor vessels, whereas endothelial PDGF-B retention is indispensable for proper integration of pericytes in the vessel wall. Our data support the notion that pericytes serve an important function in tumor vessels and highlight PDGF-B and PDGF-Rβ as promising molecular targets for therapeutic intervention.