Allele frequencies for four X-chromosomal STR were determined in a population sample of 240 unrelated males and females from the northeastern Poland by multiplex PCR and subsequent automated fluorescent detection (ABI 310) using a commercially available multiplex PCR kit (Mentype Argus X-UL). Kinship tests revealed a typical X-linked inheritance with no mutation. 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.