We have utilized detailed chromosome banding analysis to document the selection of variant genetic clones existing in cell populations obtained from a patient with ovarian adenocarcinoma. Cytogenetic analysis was performed on two samples from the patient's malignant ascites obtained over a 9-month interval. Tumor cell lines were also established in monolayer culture from each ascites sample, analyzed by chromosome banding techniques, and compared to the clonogenic population grown from each ascites specimen. Appearance of karyotypically distinct populations were identified from both the in vivo tumor and the tumor cell lines. Tumor colony-forming units (TCFU) evidenced minimal karyotypic change between the original ascites tumor and the ascites sample obtained after a 9-month period in vivo. In contrast, the cell lines derived from these two ascites samples demonstrated markedly different karyotypic populations between samples and progressive chromosome change within each sample following prolonged in vitro growth.