Expansion of a polymorphic GCC‐repeat at the FRAXE locus has been associated with expression of chromosome fragility at this site and cognitive impairment in some individuals previously testing negative for CGG‐repeat expansion in the fragile X mental retardation‐1 (FMR1) gene. To determine the frequency of FRAXE triplet repeat expansion among persons with developmental disability, 396 individuals from two institutions were studied, all of whom were negative for FMR1 repeat expansion. Clinically, there was a wide range of mental impairment, with the majority (61.1%) being severely to profoundly affected. The distribution of FRAXE GCC‐repeat numbers in the study population was 5– 38: 28 (5.6%) with 10–14 repeats; 366 (73.8%) with 15–19 repeats; 74 (14.9%) with 20–24 repeats; 20 (4.0%) with 25–29 repeats; and 5 (1.0%) with 30–38 repeats, with no individuals demonstrating repeat expansion. One profoundly retarded male was found to have a deletion of about 40 bp. Southern blots of HindIII‐digested DNAs from individuals with ≥ 26 repeats all showed normal patterns. These results suggest that FRAXE GCC‐repeat expansion is not a common cause of developmental disability in institutionalized persons with mild to profound mental retardation. © 1996 Wiley‐Liss, Inc.