We immunohistochemically examined intrathrombotic IL-6 expression using a murine model of deep vein thrombosis induced by the ligation of the inferior vena cava (IVC) and discussed the availability of intrathrombotic IL-6 for thrombus age estimation. IL-6(+) cells could be first detected at 1 day after IVC ligation in three of five samples, and all samples with postligation intervals of 3 days or more had intrathrombotic IL-6(+) cells. Thereafter, the numbers of IL-6(+) cells were elevated with the increase of postligation intervals. Double-color immunofluorescence analyses demonstrated that IL-6 was mainly expressed by intrathrombotic macrophages. In all samples with postligation intervals of 5 days or less, the IL-6/macrophage ratio (IL-6/Mϕ) was <0.5. In contrast, the IL-6/Mϕ was greater than 0.5 at ≥7 days after the IVC ligation. These observations implied that IL-6/Mϕ ratios of >0.5 would strongly indicate thrombus ages of ≥7 days. Reciprocally, the ratios of less than 0.5 would suggest thrombus ages of ≤5 days. The present study demonstrated that the immunohistochemical detection of IL-6 was suitable to estimate the age of venous thrombi.