DXS7423 and DXS8377 are two microsatellite markers located in the q28 band of chromosome X. We developed a protocol to amplify both markers in a single reaction, sequenced the most common alleles and studied allele frequencies in a Spanish population sample. DXS7423 allele variability was due to different numbers of (TCCA) repeats and five different alleles were found with apparent sizes between 181 and 197 bp. The probability of discrimination (PD) was 87% for female samples, and the expected probability of exclusion (PE) was 71%. DXS8377 appeared as a highly polymorphic marker with variable numbers of (CTC), (TCC) and (TTC) repeats. We found 18 alleles of different sizes (204-258 bp) and the PD and PE were 99% and 93%, respectively. These data suggest that DXS7423 and DXS8377 can be very useful markers for genetic forensic studies.