Primary peritoneal carcinoma (PPC) is an aggressive malignancy of the female coelomic epithelium. Previously we had analyzed 29 cases of PPC for p53 protein accumulation by immunocytochemistry. P53 was overexpressed in 83% (24 of 29) of PPCs, including 21 tumors with diffuse intense staining of 100% of tumor nuclei and three additional tumors with significant focal staining. Here we report results of a mutational analysis on the entire p53 coding sequence of 22 of these cases (comprising 18 p53-positive and four negative tumors), using single-strand conformation polymorphism (SSCP) and direct sequence analysis. Only 2 of 22 (9%) patients harbored a p53 mutation (which, interestingly, were identical and consisted of a codon 259 Asp --> His exchange), despite diffuse overexpression of high levels of nuclear p53 protein in most cases. This result indicates that (1) the abnormal p53 expression is usually not caused by mutations of the p53 gene in PPC and (2) PPC is part of a growing number of tumors that share evidence of p53 dysfunction in the absence of mutation.