The objective of this 1-year prospective follow-up study was to assess, with dual-energy X-ray absorptiometry (DXA), the effect of an anterior cruciate ligament (ACL) injury of the knee on areal bone mineral density (BMD) of the injured extremity and lumbar spine in two separate patient groups: 21 surgically treated patients (group A) and 12 conservatively treated patients (group B). Clinical and functional status of the patients and BMD of the spine (L2–L4), dominant distal radius, femoral neck, trochanter area of the femur, distal femur, patella, proximal tibia, and calcaneus of both lower extremities were determined at the time of the injury and after 4, 8, and 12 months. A surgically treated, complete ACL rupture (group A) resulted in considerable and statistically significant bone loss to the affected knee (distal femur 21%, patella 17%, proximal tibia 14%; P < 0.001 in each), whereas the other sites were clearly less affected. Patients with a conservatively treated, complete or partial ACL injury (group B) had only a small but statistically significant bone loss at the patella (−3%; P= 0.005) and proximal tibia (−2%; P= 0.022) of the injured knee, and the other sites remained unchanged. The obvious differences between the groups A and B in the severity of the injury itself (complete or partial tear), its treatment (surgical or conservative), and subsequent rehabilitation (longer nonweight-bearing times in group A) explain these different BMD results, and the forthcoming years will show whether the considerable posttraumatic osteoporosis in the affected knee of group A patients will finally recover, and if so, to what extent.