Atherogenic response to dietary fat and cholesterol challenge was evaluated in mice lacking both the LDL receptor (LDLr^−/−) and apoA-I (apoA-I^−/−) gene, LDLr^−/−/apoA-I^−/− or double-knockout mice. Gender- and age-matched LDLr^−/−/apoA-I^−/− mice were fed a diet consisting of 0.1% cholesterol and 10% palm oil for 16 weeks and compared to LDLr^−/− mice or single-knockout mice. The LDLr^−/− mice showed a 6- to 7-fold increase in total plasma cholesterol (TPC) compared to their chow-fed mice counterparts, while LDLr^−/−/apoA-I^−/− mice showed only a 2- to 3-fold increase in TPC compared to their chow-fed controls. This differential response to the atherogenic diet was unanticipated, since chow-fed LDLr^−/− and LDLr^−/−/apoA-I^−/− mice began the study with similar LDL levels and differed primarily in their HDL concentration. The 6-fold diet-induced increase in TPC observed in the LDLr^−/− mice occurred mainly in VLDL/LDL and not in HDL. Mid-study plasma samples taken after 8 weeks of diet feeding showed that LDLr^−/− mice had TPC concentrations approximately 60% of their 16-week level, while the LDLr^−/−/apoA-I^−/− mice had reached 100% of their 16-week TPC concentration after only 8 weeks of diet. Male LDLr^−/− mice showed similar aortic cholesterol levels to male LDLr^−/−/apoA-I^−/− mice despite a 4-fold higher VLDL/LDL concentration in the LDLr^−/− mice. A direct comparison of the severity of aortic atherosclerosis between female LDLr^−/− and LDLr^−/−/apoA-I^−/− mice was compromised due to the loss of female LDLr^−/−/apoA-I^−/− mice between 10 and 14 weeks into the study. Diet-fed female and, with time, male LDLr^−/−/apoA-I^−/− mice suffered from severe ulcerated cutaneous xanthomatosis. This condition, combined with a complete depletion of adrenal cholesterol, manifested in fatal wasting of the affected mice. In conclusion, LDLr^−/− and LDLr^−/−/apoA-I^−/− mice showed dramatic TPC differences in response to dietary fat and cholesterol challenge, while despite these differences both genotypes accumulated similar levels of aortic cholesterol.