Besides autosomal STR loci, markers of sex chromosomes, X and Y, are increasingly more commonly used in genetic analyses aiming at paternity testing or personal identification. The paper presents cases in which analysis of microsatellite loci of the X chromosome (X-STRs) was included in the routine examination and allowed for an unambiguous determination of the relationship between the tested individuals. The cases addressed paternity testing of female children, determination whether the examined women were paternal half-sisters, as well as personal identification of a deceased man. In none of the conducted expert opinions, the putative father's DNA sample was't available. Genotyping of X-STR markers was carried out with the use of commercial kits: Mentype Argus X-8 PCR Amplification Kit (Biotype) and Investigator Argus X-12 Kit (Qiagen).