Microsatellite instability in a subset of colorectal cancers from North America, Europe and Japan has been reported. We examined 88 colorectal cancers from Turkish patients. Five different microsatellite loci (I mono- and 4 dinucleotide repeat regions) were tested. Eight tumors displayed replication errors (RERs) with at least 2 different markers. Right-sided tumors showed significantly higher frequency of microsatellite instability compared with left-sided tumors. The frequency of RER phenotype was slightly higher in tumors occurring in younger (<50 years old) than in older patients (13% vs. 8%), and there was no association between sex and genomic instability. The frequency of genomic instability in our study group was 9%, whereas the reported frequencies in tumors from other countries varied between 12% and 16%. (C) 1996 Wiley-Liss, Inc.