Loss of heterozygosity (LOH) at the site of the retinoblastoma (RB1) gene, at 13q14, has been shown to occur in a high proportion of ovarian cancer patients. Based on this, RB1 gene inactivation was studied in primary tumor cells from 15 patients with ovarian cancer. Structural changes as well as expression of the RB1 gene were investigated. One patient had a nondisjunctional deletion at the RB1 locus, and the duplicated remaining allele carried a deletion of exon 21. In another patient, without detectable structural changes of the RB1 gene, no RB protein was detected. Allelic losses at the RB1 locus were observed in 8/13 informative cases (61%). Our results indicate that inactivation of the RB1 gene plays a role in tumor development in a minority of ovarian cancer patients. Another gene(s) on 13q also seem(s) to influence malignant transformation in patients with ovarian cancer.