It has been shown that natural killer T (NKT) cell activation accelerates atherosclerosis in apoE(-/-) mice. ApoE is, however, an important mediator in the presentation of lipids which may complicate conclusions on the role of NKT cells in atherosclerosis. Treatment of LDLr(-/-) mice with alpha-GalCer during Western-type diet feeding is therefore of interest. Atherosclerosis was induced by Western-type diet feeding and collar placement around the carotid arteries in both LDLr(-/-) and apoE(-/-) mice. Subsequently, the mice were treated twice a week with alpha-GalCer. This resulted in an 84% reduction in plaque size in LDLr(-/-) mice (P < 0.05), while no effect was observed in apoE(-/-) mice. In-vitro incubation of splenocytes with alpha-GalCer showed that LDLr(-/-) splenocytes proliferated stronger than apoE(-/-) splenocytes. This is reflected in a larger increase in production of cytokines and especially IL-10 after in-vitro stimulation with alpha-GalCer in LDLr(-/-) mice compared with apoE(-/-) splenocytes. Additionally, feeding a Western-type diet for 1.5 weeks induced a strong increase in the number of NKT cells in LDLr(-/-) mice and this increase was slower and less prominent in apoE(-/-) mice. Administration of alpha-GalCer to LDLr(-/-) mice in combination with Western-type diet feeding reduced plaque formation, but this effect was not seen in apoE(-/-) mice. This may be explained by the decreased presentation of lipids on CD1d molecules due to the lack of apoE. In this study we proved for the first time that NKT cells may also act in an atheroprotective manner.