The lactose content in dairy cows’ milk (LM) varies during an energy deficit and can represent an interesting indicator for production and health management systems. One reason of its variation is the leak of lactose in the blood (LB) also revealing the permeability status of the mammary epithelium. Up to now, the variations of LM and LB have not been studied together over lactation. Therefore, the objective was to describe the variability of LM and LB over dairy cows’ lactation. A total of 53 Holstein cows (including 17 primiparous) managed for a 16 months extended lactation were involved. The cows were divided into two feeding strategies, a control feeding strategy (NOR) and an individualized feeding strategy (EXP) during which the cows received a ration enriched in energy in early lactation (on average until week 7 of lactation). The LM and LB averages by animal were highly variable between animals (from 4.65 to 5.10% for LM, and from 18.7 to 104 mg/l for LB). The LM increased from calving to week 3 (from 4.69 to 4.89%), then kept stable, and decreased slowly from week 26 to 36 (from 4.88 to 4.79%). The LB decreased in early lactation (from 77.7 to 47.0 mg/l) and kept stable and low after week 3 until week 36 (on average 37.8 mg/l). Before week 3, the feeding strategy had no effect on LM, while between week 3 and 7 LM was higher for the EXP than the NOR cows, and it was the opposite after week 7. On average over the lactation, the EXP cows had a higher LB than the NOR cows (P=0.02). After week 3, LM was higher for the primiparous than the multiparous cows (P=0.01) while parity did not affected LB (P=0.22). Overall, over the weeks, LM and LB were not correlated. To conclude, LM and LB varied among cows, at the beginning of the lactation, and regarding the feeding treatment. In this study, parity only affected LM, and LB did not provide information to explain LM variations. It would be interesting to continue these observations in the last part of the extended lactation and relate potential health events with LM and LB concentrations.