Immunohistochemistry for keratins 5, 8, 14, and 18 was performed on Japanese Black calf thymuses at various stages of acute thymic involution. Keratins 5 and 14 were predominantly localized in the thymic medulla, while keratins 8 and 18 were broadly distributed throughout the parenchyma. Despite thymic involution, the distribution patterns of these keratins remained consistent. The cortical area, assessed by keratin 5 staining, progressively decreased with involution but retained approximately 40% of the total parenchyma even at the most severe stage. These results suggest that the thymic cortex shrinks but does not completely disappear during acute thymic involution in calves.