Forty early stage malignant and seven borderline ovarian tumours were analysed for loss of heterozygosity (LOH) on chromosomes 6, 7, 9, 11 and 17. LOH involving at least one locus was observed in 32 (80%) early stage and six (86%) borderline tumours. Frequent LOH in the early stage tumours was detected on chromosome arms 7p (31%), 7q (50%), 9p (42%) and 11q (34%) suggesting that these chromosomes harbour tumour suppressor genes which are inactivated early in tumorigenesis. Borderline tumours exhibited a similar pattern of LOH to that observed in the early stage malignant tumours, indicating that the development of both malignant and borderline forms may involve inactivation of the same set of tumour suppressor genes. Together with our previous investigation of benign ovarian tumours this data supports the theory that malignant ovarian tumours may arise from benign and borderline precursors.