Allele frequencies for four X-chromosomal STR were determined in a population sample of 240 unrelated males and females from north-eastern Poland by multiplex PCR and subsequent automated fluorescent detection (ABI 310) using a commercially available multiplex PCR kit (Mentype Argus X-UL). The genotype distributions among the females conformed with HWE for all analysed loci. The analysed quadruplex is a potential extension to a battery of autosomal systems in forensic applications, especially in the investigation of kinship analysis and deficiency cases.