We assessed the association between the haptoglobin (Hp) genotype and 2 common indicators of atherosclerotic plaque instability: macrophage infiltration and the smooth muscle cell (SMC) content. A total of 70 consecutive patients who underwent carotid endarterectomy were included in the study. For immunohistochemical study the anti-CD68 and anti-a-actin antibodies were used on adjacent serial sections; 36 plaques from patients with the Hp 1-1 or 2-1 genotype and 34 plaques from patients with the Hp 2-2 genotype were analyzed. The macrophage content (CD68+) was significantly higher in the Hp 2-2 group compared with that in the Hp 1-1 or 2-1 group (P < .001). In plaques from patients with diabetes, the SMC content was significantly lower in the Hp 2-2 group (P = .034). Carotid plaques from diabetic patients with Hp 2-2 genotype had higher macrophage infiltration and lower SMC content. Both parameters are indicators of atherosclerotic plaque instability.