Loss of heterozygosity (LOH) studies in ovarian tumors, have highlighted the chromosomal regions at 9q22-31 and 9q32-34 as being potentially important in tumor development. We have investigated LOH at 9q22-31 in 85 patients with epithelial ovarian cancer, 15 with non-epithelial tumors and 16 with benign disease. Varying patterns of LOH were observed across the markers used between different tumors, the most common (71%) being interstitial discontinuous losses. LOH was frequent, and was detected at equally high levels in malignant (71%) and benign tumors (70%). LOH occurred in epithelial invasive tumors, borderline tumors, fibromas and dermoid tumors. In malignant epithelial tumors LOH at 9q22-31 was not significantly associated with patient clinical and pathological parameters; however, survival was 29 months at the 50th centile survival, in those women whose tumors displayed LOH compared with 60 months in women whose tumors retained heterozygosity. LOH at 9q22-31 was significantly associated with LOH at the p53 locus (p=0.02) and the ovarian suppressor locus at 3p21 (p=0.05). We conclude that the chromosome region at 9q22-31, flanked by the microsatellite markers D9S1796 and D9S53, is a frequent and early event in ovarian tumorigenesis. With the of extent of discontinuous LOH, high density deletion mapping of this region using LOH as a strategy to identify candidate genes may be problematic. However with the completion of the human genome sequencing project several candidate genes are identified.