The αVβ3 integrin plays important roles in tumor-induced angiogenesis and tumor metastasis and hence, many small molecule αVβ3 ligands have been developed for cancer diagnosis and therapy. Although these show good αVβ3 targeting, most have suboptimal pharmacokinetics and show rapid tumor washout. We studied the biodistribution and tumor targeting properties of N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer based conjugates of mono-(RGDfK) and doubly cyclized (RGD4C) αVβ3 binding peptides. Endothelial cell adhesion studies showed similar affinity of HPMA-RGD4C and HPMA-RGDfK conjugate for αVβ3 integrins. Scintigraphic images of tumor bearing mice demonstrated that both conjugates showed tumor localization at 24 h post-injection and were retained at the tumor site until 192 h, whereas the efficient background clearance was observed over time. Necropsy organ counts showed that tumor accumulation of both HPMA-RGD4C and HPMA-RGDfK conjugates increased over time with peak accumulations at 4.9±0.9% and 5.0±1.2% ID/g, respectively. In contrast the background organ distribution rapidly cleared over time resulting in significant increases of tumor-to-background ratios. The radioactive dose as indicated by the area under curve (HPMA-RGD4C: 4825.3 μCi/g h and HPMA-RGDfK: 4424.9 μCi/g h) was highest for the tumor. The polymer conjugates of RGD4C or RGDfK provide a means to enhance tumor uptake, decrease background accumulation, and enable selective delivery of therapeutic or diagnostic agents to tumor sites.