Thirty-four primary, untreated sporadic breast cancers were examined for loss of heterozygosity (LOH) at tumour suppressor loci involved in colorectal cancer: APC/MCC at 5q21 and DCC at 18q21. LOH was identified in 28% informative patients at 5q21 and 31% at 18q21. LOH at 5q21 and 18q21 was compared with allele loss at 17p13 and concurrent LOH at two or more of the loci was noted in 24% of tumours. Expression of a 12 kb DCC mRNA was demonstrated in 14/34 (42%) of the cancers and in all five tumours with LOH at the DCC locus there was an additional 11 kb DCC mRNA. Abnormalities of three loci involved in colorectal cancer (5q21, 17p13 and 18q21) therefore also occur in sporadic breast cancer. The accumulation of such genetic abnormalities may confer a growth advantage important in the development of breast cancer.