Abstract. research because of the large economic impact these discoveries could have on the swine industry. The steroid 21-hydroxylase (CYP21) gene is located on chromosome 7 in the middle of the swine leukocyte antigen class (SLA) is regarded as a "candidate – gene" reproduction traits. Associations between polymorphism of the steroid 21-hydroxylase gene (CYP21) and litter size of Polish Large White × Polish Landrace sows were analysed. The 21-hydroxylase genotypes of all 286 animals were determined using a PCR-RFLP procedure. The frequencies of genotypes and alleles of CYP21/NciI and CYP21/HaeIII were follows: 0.077 – AA, 0.308 – AB, 0.615 – BB and 0.231 for CYP21/NciI^A, 0.769 for CYP21/NciI^B; 0.010 – AA, 0.420 – AB, 0.570 – BB and 0.221 for CYP21/HaeIII^A and 0.779 for CYP21/HaeIII^B. The CYP21/NciI genotype was significantly associated with the total number of piglets born, born alive and alive at weaning in the 5th – 10th parities. The sows with BB genotype had significantly (P≤0.01) higher level of traits than the animals with AB genotype. Analysis of reproductive traits in dependence on CYP21/NciI genotypes showed the statistically significant differences (P≤0.05) in number of piglets died before the day of weaned in 2-4 parities. The lowest value of this trait was found for the sows with the AB genotype (2.05%), while the highest – for the pigs with BB genotype (3.54%). Associations between CYP21/HaeIII and reproduction traits were not observed.