Fifty cancers, seven borderline tumors, and two adenomas of the ovary were immunohistochemically examined for alteration of p53 to clarify its clinical significance. Nuclear accumulation of p53 protein was detected in 60% of the cancers, but in none of the borderline tumors or adenomas. No significant correlation was found between aberrant expression of p53 and clinical stage or histological type. DNA aneuploidy was significantly more common and the Ki-67 index was significantly higher in the cancers with altered p53 protein than in those without it. There was a concordant expression level of p53 in primary and matched metastatic lesions in all twelve pairs of cancers examined. These findings suggest that alteration of p53 protein is an event that occurs in the development of cancer, but not of borderline tumors of the ovary, and that it occurs before metastasis and remains unchanged thereafter.