Restriction fragment length polymorphisms at 53 autosomal loci were screened for heterozygosity in 40 colorectal cancer patients. The DNA pattern in constitutional versus tumor/ polyp tissue was compared. More than half of the markers tested were of the VNTR (variable number of tandem repeats) type, giving the patient panel a higher informational content, since the frequency of individuals heterozygous for a particular marker is increased. Loss of alleles was revealed in 40% of the tumors from constitutionally heterozygous patients at the chromosome 17p loci, identified by the markers YNH37 and YNZ22. Similar losses were also detected on other autosomes, but at a significantly lower frequency. Our results suggest that hemi/homozygosity of 17p alleles plays a role in the development of a major subset of colorectal carcinomas. Similar observations regarding other autosomal loci may be interpreted as random losses in these tumors, or they may indicate loci important to minor clinical subclasses of colon carcinomas.