Purpose WIN55,212-2, a potent synthetic agonist of cannabinoid receptor type 1 (CB1) and cannabinoid receptor type 2 (CB2), reduces atherosclerosis in apolipoprotein E (ApoE) null mice. Although pharmacologic evidence suggests the anti-atherosclerotic effects of WIN55,212-2 are mediated via CB2, this remains to be confirmed by genetic studies. Therefore, in this study, we investigated the effects of WIN55,212-2 on development of atherosclerosis in low-density lipoprotein receptor (Ldlr) null mice with and without homozygous deletion of the CB2 gene. Methods After 6 weeks on an atherogenic diet, groups of CB2+/+ and CB2−/− Ldlr-null mice received a daily intraperitoneal injection of WIN55,212-2 or vehicle. After two weeks, plasma lipid levels and atherosclerosis in the aortic root were quantified. Results Plasma cholesterol and triglyceride levels did not differ between CB2+/+ and CB2−/− mice and WIN55,212-2 had no effect on total cholesterol levels in either genotype. However, triglyceride levels in both CB2+/+ and CB2−/− mice were significantly lowered by WIN55,212-2. The size of aortic root lesions did not differ significantly between CB2+/+ and CB2−/− mice with or without WIN55,212-2 treatment. However, WIN55,212-2 treatment significantly lowered lesional macrophage accumulation in CB2+/+ mice, and lesional smooth muscle content in both CB2+/+ and CB2−/− mice. Lesional apoptosis was also greater in CB2+/+ mice compared to CB2−/−mice, and only reduced by WIN55,212-2 in CB2+/+ mice. Collagen content and elastin fiber fragmentation were unaffected by genotype or WIN55,212-2. Conclusions WIN55,212-2 treatment does not alter lesion size in Ldlr null-mice, but does modify lesion cellularity via CB2-dependent and CB2-independent mechanisms.