Peritoneal washing cytology, widely used in the management of gynecologic malignancy, entails several difficulties in interpretation. Quantitative DNA analysis by flow cytometry (FCM) holds promise as a more objective method fo diagnosis of malignancy. We performed traditional cytologic examination and single-parameter FCM DNA analysis on peritoneal washings from 136 gynecologic laparotomies, compared these results with the final pathologic findings, and analyzed sources of error. A total of 50 laparotomies were performed for benign disease. Another 86 were performed for cervical, endometrial, and ovarian carcinomas and various other cancers. In the benign group, cytology had one false suspicious but no false positive results, and FCM showed only diploid cells. In the cancer cases, cytology had five suspicious and 13 positive results and one false negative from laboratory error. On review, 16 washings contained confirmed cancer cells. FCM, performed in 13 of these cases, was diploid in 10 and aneuploid in only 3. In six of the diploid cases, visual cell counts showed that tumor cells were present in concentrations of 2.5% or less of total cells. In the remaining four diploid cases, a second DNA determination was obtained by FCM of nuclei retrieved from paraffin blocks of the tumors. These nuclei were diploid by FCM in three of the tumors and aneuploid in only one. Single-parameter DNA FCM was too insensitive to be helpful in our material.