Copper deficiency is a mineral management disorder increasingly often diagnosed in dairy cattle herds in Poland. Diagnosis of this disorder in herds can be very difficult as subclinical deficiencies may occur over long periods of time without clear clinical symptoms. Subclinical or atypical deficiencies that long remain untreated cause major disturbances in homeostasis. Information is scarce in the literature about the changes that occur in biochemical blood parameters in the course of atypical long-term subclinical copper deficiency. This study was conducted on a group of 80 cows from farms in the central part of the Lublin region in Poland. The animals were divided into four groups: three groups with Cu deficiency (two of these groups did not receive preventive supplementation, and the third one had oral copper supplementation administered) and a control group (clinically healthy cows). Blood for the tests was collected in the same period, 6 and 12 weeks after calving and a year after the previous tests. The results obtained in the groups where no special preventive measures had been taken showed a negative influence of copper deficiency on the functional condition of organs, especially the liver and pancreas (in the final stage of the study), and on blood phosphorus concentration. Such findings were not observed in the group supplemented with a copper-rich feed additive. In this group, after only a month of being supplemented with a preparation containing copper, a 15% increase in milk yield was observed. Meanwhile, the milk yield decreased in the remaining groups.