The genetic events involved in human ovarian carcinoma development are still largely unknown. We have used DNA recombinant technologies to examine the genome of normal and neoplastic tissues from 12 different patients with such tumors. We used cloned DNA sequences homologous to loci showing restriction fragment length polymorphisms on chromosomal segments 3p, 5p, 6q, 11p, or 21q to determine the frequencies of losses of constitutional heterozygosity at these loci in tumor DNAs. Losses affecting loci on 3p, 6q, and 11p were found in a high percentage of the cases examined. In contrast, losses of heterozygosity were not found when we used DNA probes for the other 2 above chromosomal segments. Thus, genetic losses involving chromosomal segments 3p, 6q and 11p occurred at non-random frequencies in ovarian carcinomas, suggesting that inactivation of genes located on these chromosomes played a role in their development.