Hepatocellular carcinoma (HCC) is a leading cause of cancer death worldwide, reflecting incomplete characterization of underlying mechanisms and lack of early detection. Krüppel-like factor 6 (KLF6) is a ubiquitously expressed zinc finger transcription factor that is deregulated in multiple cancers through loss of heterozygosity (LOH) and/or inactivating somatic mutation. We analyzed the potential role of the KLF6 tumor suppressor gene in 41 patients who had HCC associated with hepatitis C virus (16 patients), hepatitis B virus (12 patients, one of whom was coinfected with hepatitis C virus), and other etiologies (14 patients) by determining the presence of LOH and mutations. Overall, LOH and/or mutations were present in 20 (49%) of 41 tumors. LOH of the KLF6 gene locus was present in 39% of primary HCCs, and the mutational frequency was 15%. LOH and/or mutations were distributed across all etiologies of HCC evaluated, including patients who did not have cirrhosis. Functionally, wild-type KLF6 decreased cellular proliferation of HepG2 cells, while patient-derived mutants did not. In conclusion, we propose thatKLF6is deregulated by loss and/or mutation in HCC, and its inactivation may contribute to pathogenesis in a significant number of these tumors.(HEPATOLOGY 2004; 40: 1047–1052.)