Abstract The study involved analysis of differences in bone mineral density (BMD) in the region of knee arthroplasty in 106 female patients. BMD was assessed using dual-energy X-ray absorptiometry in a follow-up time schedule: before surgery and 2 weeks postoperatively (baseline), 5, 12, 24, and 48 weeks after knee joint arthroplasty. Four assessment regions A, B, C, and D were determined in immediate proximity to the arthroplasty site. A year after surgery, a decrease in BMD was observed in all determined regions—(9.9%, 13.8%, 9.9%, and 7.6% respectively) in comparison with the baseline value. The most significant BMD decrease was observed in the period between 5 and 12 weeks after the knee joint arthroplasty. In our opinion, the decrease in BMD at the knee joint arthroplasty site is a result of the postoperative increased bone resorption and decreased patient motor activity.