Invasions of gastrointestinal nematodes in dairy cows may affect animals productivity. The most frequently detected internal parasite of dairy cattle is Ostertagia ostertagi. The objective of this study was to determine O. ostertagi invasion extensiveness in selected herds of dairy cattle, with special consideration to cows being in the first lactation, and to analyze the milk yield and contents of basic constituents of milk originating from sero-positive cows. Five herds of dairy cattle (403), with different populations of cows, were selected for the study. Invasion extensiveness in particular herds was determined and ranged from 11.9% to 27.27%. Cows being in the first lactation, the udder milk of which was shown to contain anti-O. ostertagi antibodies, were producing on average 470 kg of milk annually less than cows being in the same lactation period. The analysis of results did not confirm the statistical significance of this difference, likewise it did not demonstrate any statistically significant differences in contents of fat, protein and dry matter. Despite a lack of the statistical significance a producer suffers great economic losses. The conducted study proves that the occurrence of O. ostertagi invasion in herds of dairy cattle is a global problem and that it affects cost-effectiveness of milk production.