Pig production has always been a very important branch of economy in Croatia. Reproductive and production traits that influence on production profitability partly can be improved with animal selection. In intensive pig production the aims are breeding and production of hybrid lines, such as the Dutch hybrid Topigs, which are characterized by high reproductive ability, good feed conversion and growth of pigs. Litter size, as a low-heritable trait, is the result of interaction of many genes and environmental factors. With selection based on the length of the body, crossing of breeds and lines with the expression of heterosis effect and control of other factors (such as the optimal time for mating, feeding, as well as health status) breeders can directly increase a litter size. Inheritance of some traits, such as litter size, is difficult to determine directly. That is why molecular genetics is important in improving producting performance using genetic markers. Estrogen receptor (ESR) is the holder of superiority for litter size. It has been determined that it is associated with reproductive traits in different breeds and crossbreeds. The aim of this study was to analyze the genotypic polymorphism of ESR in 30 blood samples of sows of Topigs hybrid and reproductive performance of each genotype in the first and second litter. After DNA isolation and amplification of ESR sequence, the obtained PCR products were subjected to restriction with endonuclease PvuII, to determine the polymorphism of the target ESR segment. Restriction fragments were separated with gel-electrophoresis on 3% agarose gel. The most common genotype was the homozygote AA (segment size of 120 base pairs), and the homozygote BB (2 segments with 65 and 55 bp) was the least common. Heterozygous genotype AB had 3 segments of 120, 65 and 55 bp. It is believed that the B allele is more responsible for increasing the number of piglets born alive and total born piglets. In our study, the largest number of total born as well as stillborn piglets was in sows of genotype BB, what caused the lowest number of piglets born alive in the same group. The largest number of live born piglets was in homozygote AA. Although differences in average values of reproductive parameters between genotypes were found, they were not significant in our study.