Differences in clinical course and biological characteristics among breast cancers will probably be explained ultimately by variations in the pattern of genetic alterations among the many genes that can play roles in carcinogenesis. Loss of heterozygosity (LOH) of a particular chromosomal region in a tumor, which presumably indicates loss of a growth-regulating 'tumor-suppressor' gene in that region, may represent a useful marker for postoperative prognosis. In earlier work we observed LOH at chromosomal regions 3p14-p21 and/or 3p24-p25 in a large proportion of breast cancers. To examine whether allelic losses in either of those regions might correlate with postoperative survival, we tested tumors from a cohort of 504 breast cancer patients for allelic losses of microsatellite markers in the relevant portions of chromosome 3p. Five years postoperatively, patients whose tumors had undergone LOH at 3p24-p25 were found to have borne significantly higher risks of mortality than women whose tumors retained both alleles at that locus; i.e. the 5-year mortality rate was 22% among patients with losses at 3p24-p25 vs. 9% with retentions of heterozygosity at that locus (P=0.0014). These data indicate that LOH at 3p24-p25 is a significant predictive factor for postoperative survival of patients who have undergone surgery for breast cancer.