A detailed study was performed in 14 patients with epithelial ovarian tumours using the satellite probes 33. 15, 228S and 216S to investigate the nature of somatic changes and frequency with which clonal changes could be demonstrated during metastasis and progression. Somatic changes were evident in approximately 70% of ovarian tumours, the most common being a deletion or reduction in intensity of a band suggesting loss of heterozygosity. Additional changes that were observed included increased intensification of single bands and the appearance of novel DNA fragments. Somatic alterations were seen following digestion of DNA with methylation resistant restriction endonucleases indicating that methylation differences alone could not account for all of the somatic changes. Using DNA fingerprint analyis ovarian tumours were shown to be heterogeneous with different DNA patterns observed in different sites in five of eight patients. Generally, within an individual patient the primary and metastases appeared to share a DNA fingerprint pattern with minor variations occurring in different sites suggesting that different populations have derived from a common stem line. This study clearly demonstrates that DNA fingerprint analysis is a sensitive method to detect somatic changes in tumour DNA and for investigating the development of clonal heterogeneity in ovarian tumours.