Fifteen patients from five families with laboratory data suggesting factor X (FX) deficiency were screened for causative mutations by conformation sensitive gel electrophoresis (CSGE) followed by sequencing. All exonic and flanking intronic regions of factor X gene were amplified using PCR. After heteroduplex formation, samples were analyzed onto a polyacrylamide gel for possible mismatch. An abnormal CSGE profile indicating an heteroduplex was identified in 10/15 cases. All the 10 patients with a patter of migration suggesting a mismatch had a laboratoristic pattern of FX deficiency whereas the five cases with a normal CSGE aspect referred to the normal components of the families who did not carry any FX defect. Sequencing demonstrated that the 10 exons, which showed a suspect CSGE pattern, harbored a mutation responsible for the factor X defect. Of the five mutation identified, two were recognized to be novel mutations (a 871C>T substitution and a 1169G>T transversion in exon 8), both located in the catalytic portion of FX. CSGE may be an effective and simple procedure for screening factor X gene mutations.