Dietary factors promote obesity and obesity‐related disorders, such as fatty liver disease. Natural killer T (NKT) cells are components of the innate immune system that regulate proinflammatory (Th‐1) and anti‐inflammatory (Th‐2) immune responses. Previously, we noted that NKT cells are selectively reduced in the fatty livers of obese, leptin‐deficient ob/ob mice and demonstrated that this promotes proinflammatory polarization of hepatic cytokine production, exacerbating lipopolysaccharide (LPS) liver injury in these animals. In the current study, we show that hepatic NKT cells are also depleted by diets that induce obesity and fatty livers in wild‐type mice, promoting Th‐1 polarization of hepatic cytokine production and sensitization to LPS liver injury despite persistent leptin. Adult male C57BL6 mice fed diets containing high amounts of either fat or sucrose, or combined high‐fat, high‐sucrose, develop increased hepatic NKT cell apoptosis and reduced liver NKT cells. The hepatic lymphocytes are more Th‐1 polarized with increased intracellular interferon gamma and tumor necrosis factor alpha. Mice fed high‐fat diets also exhibit more liver injury, reflected by 2‐fold greater serum alanine aminotransferase (ALT) than control animals after receiving LPS. In conclusion, when otherwise normal mice are fed with high‐fat or sucrose diet, they become obese, develop fatty livers, and acquire hepatic innate immune system abnormalities, including increased NKT cell apoptosis. The latter reduces liver NKT cell populations and promotes excessive hepatic production of Th‐1 cytokines that promote hepatic inflammation. These diet‐induced alterations in the hepatic innate immune system may contribute to obesity‐related liver disease. (HEPATOLOGY 2005;42:880–885.)