Microsatellites are short repetitive nucleotide sequences that, through mutation, can undergo either expansion or contraction. This novel mutational mechanism known as microsatellite instability may play a role in carcinogenesis. We investigated the incidence of microsatellite instability in a series of primary breast carcinoma surgical specimens.Using polymerase chain reaction techniques followed by polyacrylamide/urea gel electrophoresis, we analyzed 46 pairs of normal and primary breast tumor samples at seven different microsatellite loci, five of which were located on chromosome 17.Thirteen of our 46 tumors (28.2%) demonstrated microsatellite instability. Five tumors (10.8%) were unstable at two or more loci, and of those, four (8.7%) were unstable at different loci on different chromosomes. An additional five tumors demonstrated loss of heterozygosity alone when compared with their normal counterparts.These findings indicate that microsatellite instability is present in primary breast cancer populations and, although the mechanism of action has yet to be elucidated, may play a role in breast carcinogenesis.