Fragile X syndrome is the most frequent cause of inherited mental retardation. The phenotype in this syndrome is quite variable and less conspicuous in younger patients, making clinical diagnosis difficult and thus making molecular diagnosis necessary. The use of clinical checklists in mentally retarded individuals can help selecting patients to be given priority in the molecular investigation for the fragile-X mutation in the FMR1 gene. We evaluated two clinical checklists in a sample of 200 Brazilian male patients with mental retardation. The highest scores in the two checklists concentrated among the 19 males (9.5%) found to carry full mutations. Our results confirm the importance of fragile-X checklists as a clinical tool in the study of mentally retarded patients.