Introduction: The human leukocyte antigen system, also known as the major histocompatibility complex, has long gone been described in literature, as one of the most researched systems in the field of genetics.. It has its significance in that it encodes molecules responsible for the regulation of immune processes and the presentation of endogenous and exogenous antigens to immune cells (mainly T lymphocytes). It is located on the short tail of the chromosome 6 and is divided into three classes, which are then divided into loci. It makes up for 1% of the entire human genome. The HLA - B * 27 gene, which is a class I gene in the HLA system, has previously been linked to a numerous autoimmune diseases, primearly to reumatoid diseases. Methods: In this bachelor thesis, the frequency of the most common subtypes of the HLA - B * 27 gene in the population of the Split - Dalmatia County was analyzed with the usage of the basic statistical formulas and direct counting. The methods used to detect HLA system genes are serological and molecular. Serological testing includes a microlymphocitotoxicity test, and molecular testing includes PCR-SSO and PCR-SSP methods. Results : The HLA - B * 27: 02 subtype was present in the largest number of our patients, with a frequency of 43%, followed by HLA - B * 27: 05 with a slightly lower frequency of 38%. The otherwise rarer allele, HLA - B * 27: 03, was observed in our patients in a slightly higher number, with a frequency of 18%, and an extremely rare allele HLA - B * 27: 08 was found in one patient (1%). Disscusion: By studying the frequencies of established alleles of the HLA - B * 27 gene in world populations, we can say that the frequency of the most observed HLA - B * 27: 02 and HLA - B * 27: 05 alleles within our sample is equal to the frequencies of these same subtypes in Europe. The HLA - B * 27: 03 subtype was found in our sample in more patients than expected, as its frequency is higher in some other parts of the world (North America) than in Europe. The HLA - B * 27: 08 subtype was observed in one patient, and observing its worldwide distribution, it has a small representation in Europe, especially in Spain. Conclusion: The frequency of the two most common subtypes of the HLA - B * 27 gene in subjects in the Split - Dalmatia County does not differ much from the global frequency of the most common subtypes.