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Background & Aims Levels of the Golgi protein 73 (GP73) increase during development of hepatocellular carcinoma (HCC); GP73 is a serum marker for HCC. However, little is known about the mechanisms or effects of GP73 during hepatic carcinogenesis. Methods GP73 was overexpressed from a retroviral vector in HepG2 cells, which were analyzed in proliferation and migration assays. Xenograft tumors were grown from these cells in nude mice. The effects of monoclonal antibodies against GP73 were studied in mice and cell lines. GP73 −/– , GP73 +/− , and GP73 +/+ mice were given injections of diethylnitrosamine to induce liver injury. Levels of GP73 were reduced in MHCC97H, HCCLM3, and HepG2.215 cell lines using small hairpin RNAs; xenograft tumors were grown in mice from MHCC97H–small hairpin GP73 or MHCC97H-vector cells. We used microarray analysis to compare expression patterns between GP73-knockdown and control MHCC97H cells. We studied the effects of the mechanistic target of rapamycin (mTOR) inhibitor rapamycin on GP73 expression in different cancer cell lines and on growth of tumors in mice. Levels of GP73 and activated mTOR were quantified in human HCC tissues. Results Xenograft tumors grown from HepG2 cells that expressed GP73 formed more rapidly and more metastases than control HepG2 cells in mice. A monoclonal antibody against GP73 reduced proliferation of HepG2 cells and growth of xenograft tumors in mice. GP73 −/– mice had less liver damage after administration of diethylnitrosamine than GP73 +/− or GP73 +/+ mice. In phosphatase and tensin homolog–null mouse embryonic fibroblasts with constitutively activated mTOR, GP73 was up-regulated compared with control mouse embryonic fibroblasts; this increase was reversed after incubation with rapamycin. Expression of GP73 also was reduced in HCC and other cancer cell lines incubated with rapamycin. mTORC1 appeared to regulate expression of GP73 in cell lines. Activated mTOR correlated with the level of GP73 in human HCC tissues. Injection of rapamycin slowed the growth of xenograft tumors from MHCC97H-vector cells, compared with MHCC97H–short hairpin GP73 cells. Conclusions Increased expression of GP73 promotes proliferation and migration of HCC cell lines and growth of xenograft tumors in mice. mTORC1 regulates the expression of GP73, so GP73 up-regulation can be blocked with rapamycin. mTOR inhibitors or other reagents that reduce the level or activity of GP73 might be developed for the treatment of HCC.