Bone mineral density (BMD) and clinical status of 40 patients with a chronic, unilateral patellofemoral pain syndrome (PFPS) were determinated. The mean duration of the disease at the time of the follow-up was 7.6 +/- 1.8 (SD) years. The BMD was measured at the spine (L2-L4), and the femoral neck, trochanter area of the femur, distal femur, patella, proximal tibia, and calcaneus of both lower extremities using a dual-energy X-ray absorptiometric (DXA) scanner. The mean BMD of the affected limb (compared with the unaffected side) was significantly lower in the distal femur (-3.3%; P = 0.002), patella (-2.5%; P = 0.016), and proximal tibia (-1.9%; P = 0.008). The femoral neck, trochanter area of the femur, and calcaneus showed no significant side-to-side differences, and the spinal BMDs of men and women with the PFPS were comparable with the manufacturer's age-adjusted reference values for Western European men and women. The relative BMDs of the affected knee showed strongest correlation with the muscle strength of the same knee: the better the muscle strength compared with the healthy knee, the higher the relative BMD (r = 0.56-0.58 with P < 0.001 in each anatomic site of the knee). In the stepwise regression analysis, low body weight or low body mass index, high level of physical activity, the patient's good subjective overall assessment of his/her affected knee, and short duration of the symptoms were also independent predictors of the high relative BMD in the affected knee so that along with the muscle strength these variables could account for 51% of the variation seen in the relative BMD of the femur, 61% in the patella, and 54% in the proximal tibia. In conclusion, chronic patellofemoral pain syndrome results in a significantly decreased BMD in the knee region of the affected limb. The spine, proximal femur, and calcaneus are not affected. Recovery of normal muscle strength and knee function seems to be of great importance for good BMD.