Abstract Background: A phase I dose-escalating study of pazopanib was conducted to determine the maximum tolerated dose (MTD), pharmacokinetic/pharmacodynamic relationships, and clinical activity in patients with advanced hepatocellular carcinoma (HCC). Experimental Design: Asian patients (N = 28) were dose escalated on pazopanib (200–800 mg) once daily (QD) on 21-day cycles, with MTD as the primary endpoint using a modified 3 + 3 design. Changes in tumor vasculature were evaluated by dynamic contrast-enhanced MRI (DCE—MRI). Results: Two of five patients at the 800-mg dose level experienced dose-limiting toxicities [grade 3 aspartate aminotransferase (AST)/alanine aminotransferase (ALT) elevations and grade 3 malaise]. The MTD in patients with HCC (Child–Pugh class A) was 600 mg QD. Diarrhea, skin hypopigmentation, and AST elevation were the most commonly reported adverse events at the MTD. Mean Cmax and area under the concentration-time curve (AUC0–6) of pazopanib and its metabolites did not increase dose proportionally across the 200 to 800 mg range. Reductions in IAUGC and Ktrans were shown at all pazopanib doses evaluated, with the greatest reductions at 600 and 800 mg. Although larger DCE-MRI parameter decreases were associated with larger C24 and Cmax values, there was no constant relationship between tumor perfusion decreases measured by DCE-MRI and plasma pazopanib pharmacokinetic parameters. Overall, 19 patients (73%) had either partial response or stable disease. Conclusion: Pazopanib has a manageable safety profile in patients with advanced HCC, and 600 mg was chosen for further development of pazopanib in advanced HCCs. Moreover, pazopanib reduced tumor vessel leakage, as shown by DCE-MRI, indicating a direct effect on HCC vasculature that might be associated with its antitumor activity. Clin Cancer Res; 17(21); 6914–23. ©2011 AACR.