p53 protein expression has been studied in epithelial inclusion cysts adjacent to and contralateral to serous carcinoma of the ovary and compared to epithelial inclusion cysts associated with borderline tumours and in normal ovaries. Atypia was found in epithelial inclusion cysts in eight of the thirteen advanced (stage III) serous ovarian carcinomas. Of these eight with atypical epithelial inclusion cysts, five showed immunoreactivity for p53. In the borderline tumours and normal ovaries no atypia in such cysts was found. p53 expression was also seen more frequently in surface epithelium associated with ovarian serous adenocarcinoma (ten of 13) than in normal ovaries (one of 13). We postulate that focal areas of atypia (ovarian intra-epithelial neoplasia) in epithelial inclusion cysts are the precursors of ovarian malignancy. In some cases, at least, p53 protein expression may precede overt cytological abnormalities.