The study on polymorphism within the STAT5A gene (transition C6853T) was conducted using the PCR-RFLP method and AvaI restrictase. The study covered a herd of 723 cows of the Polish Red-and-White variety of Holstein Friesian breed, kept for dairy purposes in the Opole region, Poland. Two alleles (C and T) of the analyzed STAT5A polymorphism were found in the studied herd. The alleles determined the occurrence of two genotypes: CC and CT. The homozygous TT genotype was not found. The STAT5A/AvaI allele frequencies were as follows: C—88.31% and T—11.69%, whereas the genotype frequencies were 76.6% for CC and 23.4% for CT. The analysis of associations between the STAT5A/AvaI polymorphism and milk utility traits considered in the study showed that these traits were different in animals with different STAT5A/AvaI genotypes.