['Knee pain relief has been suggested to potentially alter radioanatomic positioning in conventional standing antero-posterior knee radiographs. This study was performed to determine whether this is always the case and in particular if it applied to two recent randomised, placebo-controlled trials showing both symptom- and structure-modification with glucosamine sulfate in knee osteoarthritis.', 'Patients in the two studies were selected if they completed the 3-year evaluations and, irrespectively of treatment, (1) were pain-improvers in that they underwent Western Ontario and McMaster Universities (WOMAC) osteoarthritis index (WOMAC) pain decrease at least equal to the mean improvement observed with glucosamine sulfate, or (2) if their baseline standing knee pain (item #5 of the WOMAC pain scale) was "severe" or "extreme" and improved by any degree at the end of the trials. Changes in minimum joint space width were then compared between treatments.', 'Knee pain was of mild-to-moderate severity in the two original studies and in all patient subsets identified here. Obviously, there were more pain-improvers in the glucosamine sulfate than in the placebo subsets (N=76 vs 57 in pooling the two studies), but WOMAC pain scores improved to the same extent (over 50% relative to baseline). Notwithstanding such a major pain relief, patients in the placebo subsets of both studies suffered a definite mean (SE) joint space narrowing, that was of -0.22 (0.15)mm in the pooled analysis, and that was not observed with glucosamine sulfate: +0.15 (0.07)mm; P=0.003. Similar evidence was found in the smaller subsets with at least severe baseline standing knee pain improving after 3 years.'] Is assessment of joint space narrowing with conventional standing antero-posterior radiographs : relief in mild-to-moderate pain a confounder in recent osteoarthritis structure-modifying drug trials?