Familial erythrocytosis is a rare congenital disorder with heterogenous genetic background, defined by increased red blood cell number, elevated haematocrit and haemoglobin in blood. Hypoxia-inducible factors alpha (HIFA) activate transcription of target genes in low-oxygen conditions and consequently upregulate red blood cell production. Sequence variants in gene for one of three HIFA isoforms, endothelial PAS domain protein 1 (EPAS1), lead to the development of familial erythrocytosis type 4. In collaboration with the Specialized haematological laboratory of the Clinical department of haematology, University Medical Centre Ljubljana, we established diagnostic test for identification of EPAS1 gene variants associated with erythrocytosis. We optimized PCR reaction for amplifiqation and sequenced three selected regions, with total length of 1374 base pairs, using Sanger method. Newly developed diagnostic method was used to analyse samples of four patients with unknown eythrocytosis from three different families and two healthy individuals from control group. All four patients were homozygous (GG) for sequence variant c.C1035-7G (rs7557402) and three patients were homozygous (TT) for sequence variant c.C1249+151 (rs11678817). Related individual from control group was heterozygous, whereas unrelated individual from control group was homozygous (CC) at both loci. To elucidate the effect of identified variants on development of erythrocytosis, additional research is needed in the future.