Most hereditary nonpolyposis colorectal cancer (HNPCC) patients inherit a defective allele of a mismatch repair (MMR) gene, usually MLH1 or MSH2, resulting in high levels of microsatellite instability (MSI-H) in the tumors. Presence of MSI in the normal tissues of mutation carriers has been controversial. Here we directly compare MSI in the peripheral blood leukocyte (PBL) DNA of seven HNPCC patients carrying different types of pathogenic MMR mutations in MLH1 and MSH2 genes with the PBL DNA of normal age-matched controls and of patients with sporadic colorectal cancer (SCRC). Small pool PCR (SP-PCR) was used studying three microsatellite loci for at least 100 alleles each in most samples. The average frequencies of mutant microsatellite fragments in each HNPCC patient (0.04-0.24) were significantly higher (po0.01) relative to their age-matched normal controls with mutant frequencies (MF) from 0.00 to 0.06, or SCRC patients (MF from 0.01-0.03). The data support the conclusions that higher MF in the PBL DNA of HNPCC patients is real and reproducible, may vary in extent according to the type of germline MMR mutation and the age of the individual, and provide a possible genetic explanation for anticipation in HNPCC families.