Several studies have suggested that increased subchondral bone turnover is a determinant of progression of osteoarthritis (OA). To test this hypothesis, the level of urinary N-terminal type I collagen telopeptides (NTx) and C-terminal type I collagen telopeptides (CTx), which are validated markers of bone resorption, was measured at 3 different time points in a subset of patients from the Chingford study. The original Chingford study population comprised 1,003 women. From this group, postmenopausal women not receiving any bone-modifying medication who had a baseline knee radiograph and a repeat radiograph 4 years later, and for whom a baseline lumbar spine bone mineral density (BMD) measurement was available, were identified and separated into 4 groups as follows: controls (n = 50), progressive OA (n = 71), nonprogressive OA (n = 36), and osteoporosis (n = 59). NTx and CTx were measured in urine samples collected at baseline, year 1, and year 2. Patient age and years since menopause were similar among groups at baseline. As expected, both body mass index (BMI) and BMD were lowest in patients with osteoporosis. Median resorption marker levels over the 3 time points were 31-87% higher in patients with either progressive OA or osteoporosis than in controls and patients with nonprogressive OA (P < 0.01, except for levels of CTx in patients with progressive OA versus nonprogressive OA). Levels of NTx and CTx did not differ significantly between women with progressive OA (defined either by the presence of osteophytes or by joint space narrowing) and those with osteoporosis or between controls and women with nonprogressive OA. Results were essentially unchanged after adjustment for age, BMI, BMD, and past use of hormone replacement therapy, or when NTx and CTx values at each time point were analyzed separately. Our data demonstrate that bone resorption is increased in patients with progressive knee OA and is not increased in those with nonprogressive knee OA. The increase in bone resorption seen in patients with progressive knee OA is similar to that observed in patients with osteoporosis. Altered bone turnover may be a diagnostic or therapeutic target in patients with progressive OA.