The aim of the study was to investigate the effect of BCS (Body Condition Score) on the calving day and its decrease during early lactation on the biochemical blood indices and reproductive performance. One hundred and thirty-one Polish Holstein-Friesian cows were divided into three groups according to the North-American body condition score on the calving day (AC≤3.5 point BCS; MID 3.51-3.75 point BCS; FAT>3.75 point BCS) and according to the decrease in BCS during early lactation (HG >0.49 point BCS; WEL 0.49-0.25 point BCS, L<0.25 point BCS). In current study, significant interaction between change of BCS during early lactation and time of blood sampling on BHBA concentration was observed. In the AC group (≤3.5 BCS), the highest concentrations of glucose on 3 and 5 d of lactation compared to the MID and FAT groups and of insulin on 28 d and also IGF-I on 5 and 28 d of lactation compared to the FAT group were recorded. In the FAT group, the highest concentration of NEFA on 3 and 5 d compared to the AC group and of BHBA on 28 d of lactation compared to the MID group was recorded. The body condition score on the parturition day affected the reproductive performance; in the FAT group (>3.75 BCS) the lowest conception rate of the first insemination, insemination index and thereby the longest days open were observed. The highest decreases in BCS (HG >0.49 points) resulted in increased concentrations of NEFA on 3 d and of BHBA on 3 and 5 d. Moreover, in the HG group, the lowest concentration of T3 on 3, 5 and 28 d as compared to the WEL group was recorded. We concluded that the BCS (>3.5 points) on the calving day had a significantly negative effect on the metabolic status of dairy cows in the postpartum period estimated by the concentration of biochemical blood indices characterising carbohydrates (IGF-I, insulin) and lipid (NEFA, BHBA) metabolism and also the reproductive performance such as the conception rate of the first insemination, insemination index, days open. We suggested that the blood serum concentrations of IGF-I and NEFA were the most sensitive biochemical markers of the metabolic status of dairy cows in our study.