To determine whether genetic abnormalities present in primary ovarian tumors can be used to detect cancer cells in peritoneal fluid, we tested 14 ovarian cancers and 1 benign tumor of the ovary for loss of heterozygosity (LOH) at chromosomal arms 13q, 17p, 17q, and 22q and for mutations in the p53 and K-ras genes. In each case, matched primary tumor, normal tissue, and peritoneal fluid were analyzed. The highest frequency of LOH was found on chromosomal arm 17p (42%), followed by chromosomal arm 17q (36%), 22q (30%), and 13q (21%). Identical alterations were detected in matched peritoneal fluid (either peritoneal wash or ascitic fluid) in 3 of the 8 patients with LOH in the tumor (38%). Direct sequence analysis detected p53 mutations in 3 of the 14 malignant tumors (21%) and no (0) K-ras mutations. Identical mutations were detected in matched peritoneal fluid from all 3 patients with p53 mutations. All 8 of the 14 (57%) malignant tumors that showed at least one genetic abnormality were serous adenocarcinoma and identical alterations were detected in 5 of the 8 (62%) matched peritoneal fluid samples. Our findings indicate that molecular abnormalities can be detected in peritoneal fluid from patients with ovarian cancer and may be used to complement current conventional diagnostic procedures for detection of primary ovarian cancer.