The evaluation of the effect of supplements of conjugated linoleic acid (CLA) given to pregnant and lactating sows on the phenotypic manifestation of their reproductive performance and on the birth and weaning weight of piglets was performed under conditions of a commercial herd of pigs. The experiment involved altogether 40 hybrid sows divided into two equal groups, i.e. Controls with 20 sows and Experimental Group with also 20 sows. Experimental Group received within the time interval from the 108th day of pregnancy to the 28th day after the parturition a supplement of 2 % of CLA. In individual litters the following performance parameters were recorded: number of all newborn piglets, number of piglets born alive, number of stillbirths, number of weaned piglets and individual weights of newborn piglets and weaned piglets. Obtained results concerning numbers of all newborn piglets in Controls and Experimental Group (i.e. 12.20 ± 2.09 and 11.85 ±2.58, respectively) indicate that supplementing of the feeding ration with CLA in the last stage of pregnancy does not influence the size of litter. In Controls, numbers of piglets born alive were higher by 0.4 animals per litter; this difference, however, was statistically insignificant. Also the weights of newborn and weaned piglets were not influenced by the supplement of CLA. Statistically significant differences (P ≤ 0.01) were found out between Controls and Experimental Group in numbers of weaned piglets, viz. 9.75 ± 1.68 vs. 8.35 ± 1.39, respectively, and between corresponding losses until weaning, viz. 0.95± 1.39 vs. 2.60 ± 1.93 in Controls and Experimental Group, respectively (P ≤ 0.01).