Osteoarthritis (OA) of the knee is common in the aging population. In patients with OA, bone mineral density (BMD) is usually increased, but the fracture rate does not appear to be systematically lower than in age-matched healthy controls. The aim of our study was to describe hip BMD in patients presenting with unilateral symptomatic knee OA. Patients with painful knee OA were prospectively included in a single-center, randomized, double-blind, placebo-controlled clinical trial to evaluate the structure-modifying efficacy of an oral chondroitin sulfate treatment on the knee joint. The majority of these patients underwent additional measurements of BMD of their lumbar spine and both hips using dual-energy X-ray absorptiometry (DXA). The hip BMD values of the leg with symptomatic knee OA were compared with the contralateral hip. One-hundred and sixty-one patients (81 men and 80 women; aged 62.6 +/- 9.2 yr, range 40-82 yr) underwent DXA. The median total hip BMD was higher than in age-matched controls, but patients had a relatively lower hip BMD in the knee OA-affected leg (p = 0.001). Our knee OA patients rarely presented with concomitant osteoporosis, but usually had a relatively lower hip BMD on the affected leg. Therefore, we suggest that the hip of the leg with symptomatic knee OA should be measured if DXA is acquired only at one hip. Future studies have to assess whether the relative decrease of BMD at the hip of the leg with knee OA might influence fracture incidence.