The allelic variation of the FMR1 CGG repeat was investigated by small-pool PCR in nonneoplastic peripheral blood leukocytes from HNPCC patients and matched controls for similar CGG repeat lengths. The allelic variation for repeat lengths appears to be roughly twice as frequent in HNPCC patients as in controls, especially when patients are mutated in hMLH1. There are more expansions in HNPCC patients (42%) than in controls (20%) but this difference is statistically borderline. The mean length of expansions relative to the genuine size did not differ in HNPCC patients or controls (respectively 17% and 20% of the constitutional allelic length). The reported data suggest that instability within nonneoplastic cells of a subset of HNPCC patients might be one mechanism for transition from normal to the premutation range of the FMR1 CGG repeat.