The p8 protein is involved in various cells under cellular stress, such as apoptotic stimuli. In this study, p8 protein expression was investigated in human medullary thyroid carcinoma. in order to elucidate its physiological significance. We immunohistochemically investigated p8 expression in 69 cases of medullary thyroid carcinoma. Thirty cases (43.4%) were regarded as a high group of p8 expression with a labelling index of 50% or higher. The p8 expression level was directly linked to lymph node metastasis, extrathyroid extension and UICC stage. Furthermore, there were no cases showing low p8 expression, but 24.8% of those with high p8 expression showed recurrence. However, p8 expression was not linked to the apoptotic index. These findings suggest that p8 expression plays an important role in the progression of medullary carcinoma, but does not have an anti-apoptotic function.