Recent reports have shown allele loss at the c-Ha-ras1 locus on the short arm of chromosome 11 in some types of tumors. To determine whether loss of heterozygosity occurs at the c-Ha-ras1 locus in uncultured human ovarian carcinomas we used Southern blot analysis to study DNA from 17 pairs of ovarian tumors and matched white blood cell samples from the same patients. In one of these 17 tumors, the c-Ha-ras1 locus was rearranged, and in five tumor DNAs from ten informative patients, a c-Ha-ras1 allele was lost. This loss, of relatively high incidence, appears to be an important characteristic of human ovarian cancer and may provide a useful tool for understanding its biological behavior.