Background: A major determinant of the risk of myocardial infarction is the stability of the atherosclerotic plaque. Macrophage-rich plaques are more vulnerable to rupture, since macrophages excrete an excess of matrix-degrading enzymes over their inhibitors, reducing collagen content and thinning the fibrous cap. Several genetic studies have shown that disruption of signalling by the chemokine monocyte chemoattractant protein 1 reduced the lipid lesion area and macrophage accumulation in the vessel wall. Methods: We have tested whether a similar reduction in macrophage accumulation could be achieved pharmacologically by treating apolipoprotein-E-deficient mice with the chemokine inhibitor NR58-3.14.3. Results: Mice treated for various periods of time (from several days to 6 months) with NR58-3.14.3 (approximately 30 mg/kg/day) consistently had 30–40% fewer macrophages in vascular lesions, compared with mice treated with the inactive control NR58-3.14.4 or PBS vehicle. Similarly, cleaved collagen staining was lower in mice treated for up to 7 days, although this effect was not maintained when treatment time was extended to 12 weeks. The vascular lipid lesion area was unaffected by treatment, but total collagen I staining and smooth muscle cell number were both increased, suggesting that a shift to a more stable plaque phenotype had been achieved. Conclusions: Strategies, such as chemokine inhibition, to attenuate macrophage accumulation may therefore be useful to promote stabilization of atherosclerotic plaques.