An association has previously been reported between exposure to medical diagnostic ionizing radiation and papillary thyroid cancer in women. To further evaluate potential mechanisms in carcinogenesis, the expression of p53, c-erbB-2, as well as Ki-67, estrogen and progesterone receptors were analyzed by immunohistochemistry in 19 women exposed to X-rays and for comparison in nine women without such reported exposure. They all had papillary thyroid cancer. No difference was found between these groups. The results of this study showed that p53, c-erbB-2, Ki-67, estrogen and progesterone receptors are not involved in papillary thyroid cancer associated with exposure to medical diagnostic ionizing radiation.