The Golden Syrian hamster (Mesocricetus auratus) has been shown to be a useful model of both human lipoprotein metabolism and the development of atherosclerosis. We report the effects of dietary lipids on the progression and regression of atherosclerosis in this model. In the first study, hamsters fed on coconut oil (150 g/kg diet) and cholesterol (30 g/kg diet) developed lipid-rich lesions in the ascending aorta (0⋅28 (SD 0⋅14) mm 2 ) and aortic arch (0⋅01 (SD 0⋅01) mm 2 ) after 4 weeks that continued to progress over the next 8 weeks (0⋅75 (SD 0⋅41) mm 2 and 0⋅12 (SD 0⋅11) mm 2 for the ascending aorta and aortic arch respectively). Removal of cholesterol from the diet halted this progression. Furthermore, in animals fed on olive oil in the absence of added cholesterol, plasma LDL-cholesterol concentrations were lower (P Ͻ 0⋅05) and the extent of atherosclerotic lesions was reduced (P Ͻ 0⋅001 for both regions of the aorta) compared with animals fed on coconut oil (with no added cholesterol). In a second study, animals were fed on the atherogenic diet for 10 weeks, transferred to diets containing either coconut oil (150 g/kg diet) or olive oil (150 g/kg diet) without added cholesterol and monitored for up to 16 weeks. In the ascending aorta, lesion size doubled in animals fed on coconut oil but stabilized in those fed on olive oil. In the aortic arch, lesion size decreased linearly (P Ͻ 0⋅05, P Ͻ 0⋅001 for coconut oil and olive oil respectively) with the greatest reduction being seen in the olive-oil-fed animals (P Ͻ 0⋅05). Again, progression and regression of atherosclerosis appeared to reflect the relative concentrations of LDL-cholesterol and HDL-cholesterol in the plasma. We conclude that the male Golden Syrian hamster represents a useful model of dietary induced regression as well as progression of atherosclerosis. Hamster: Atherosclerosis: Coconut oil: Olive oil: Dietary fatThe Golden Syrian hamster (Mesocricetus auratus) has been used extensively in studies of lipoprotein metabolism.