Chromosome 11p15 has attracted considerable attention because of the biological importance of this region to human disease. Apart from being an important tumor suppressor locus showing loss of heterozygosity (LOH) in several adult and childhood cancers, 11p15 has been shown by linkage analysis to harbor the gene(s) for the Beckwith-Wiedemann syndrome. Furthermore, the clustering of known imprinted genes in the 11p15.5 region suggests that the target gene may also be imprinted. However, positional cloning efforts to identify the target genes have been complicated by the large size (∼10 Mb) and complexity of LOH at 11p15. Here, we have analyzed 94 matched normal and breast tumor samples using 17 polymorphic markers that map to 11p15.5-15.4. We have defined precisely the location of a breast tumor suppressor gene between the markers D11S1318 and D11S4088 (∼500 kb) within 11p15.5. LOH at this region occurred in ∼35–45% of breast tumors analyzed. In addition, we have fine-mapped a second, critical region of LOH, that spans the markers D11S1338-D11S1323 (∼336 kb) at 11p15.5-p15.4, that is lost in ∼55–60% of breast tumors. There is a striking correlation between the loss of the two 11p loci and the clinical and histopathological features of breast tumors. LOH at region 1 correlated significantly (P = 0.016) with early events in malignancy and invasiveness. In contrast, the loss of the more proximal region 2, is highly predictive (P = 0.012) of aggressive metastatic disease. Thus, two distinct tumor suppressor loci on chromosome 11p15 may contribute to tumor progression and metastasis in breast cancer. The fine mapping of this intriguing chromosomal region should facilitate the cloning of the target genes and provide critical clues to understanding the mechanisms that contribute to the evolution of adult and childhood cancers.