Cytogenetic studies of ovarian cancer have been conducted in the Medicine Branch, NCI, National Institutes of Health for 5 years. A total of 72 patients were studied by direct preparation and/or 1- to 3-day short-term culture of ascites (86 samples), pleural fluid (4 samples), and tumor (2 samples). Repeat examinations (1-24 months later) were performed in 7 of the 72 patients. Forty-four patients (62%) were successfully analyzed with banding techniques: 6 patients had adenocarcinoma, 7 had serous adenocarcinoma, 13 had serous papillary adenocarcinoma, 7 had serous papillary cystadenocarcinoma, 2 had mucinous adenocarcinoma, 6 had undifferentiated or poorly differentiated adenocarcinoma, 1 had clear cell adenocarcinoma, and 2 were not classified. Of these 44 patients, 29 had received prior chemotherapy, 14 were untreated, and in 1 patient the treatment status was unknown. Aneuploidy was observed in all patients and there was considerable variation in the chromosome numbers (even within single samples), often ranging from diploidy to triploidy to tetraploidy. All 44 patients had numerical abnormalities and 39 had structural abnormalities. The chromosomes most frequently involved in structural abnormalities (in decreasing order according to the number of patients involved) were #1, #3, #2, #4, #9, #10, #15, #19, #6, and #11; the least involved chromosomes were #21 and #5. Clone formation and the number of chromosomes involved in structural abnormalities increased with duration of disease and were more extensive in patients treated with chemotherapy than in patients treated with surgery alone. Our data did not show a deletion of chromosome #6 (6q-) to be specific for ovarian cancer.