[en] Objective: To assess whether improvement in knee pain biased the determination of the structure-modifying effect reported for glucosamine sulfate in two recent 3-year, randomised, placebo-controlled clinical trials, in which conventional standing antero-posterior full extension knee radiographs were used for the measurement of joint space narrowing, and in which pain relief might have improved knee full extension. Design: Patients completing the 3-year treatment course were selected based on a WOMAC pain decrease at least equal to the mean improvement in the glucosamine sulfate arms in either of the original studies, irrespective of treatment with glucosamine sulfate or placebo (drug responders or placebo responders). In a second approach, 3-year completers were selected 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. In both cases, changes in minimum joint space width were compared between treatment groups. Results: Global knee pain was rnild-to-moderate in the two study populations and in all patient subsets identified. There were obviously more pain improvers in the glucosamine sulfate subsets (N=76 in the two studies combined) than in the placebo subsets (N=57), but WOMAC pain scores improved to the same extent, which was as large as over 50% relative to baseline. Nevertheless, the placebo subsets in both studies underwent an evident mean (SD) joint space narrowing, which in the pooled analysis of both studies was -0.22 (0.80) mm, and was not observed with glucosamine sulfate: +0.15 (0.60) mm (P=0.003 vs placebo). Similar results were found in the smaller subsets with greater than or equal to severe baseline standing knee pain that improved after 3 years, with a joint space narrowing nevertheless of -0.28 (0.76) mm with placebo (N=26), not observed with glucosamine sulfate: +0.21 (0.68) mm (N=31; P=0.014 vs placebo). Conclusions: Knee pain relief did not bias the report of a structure-modifying effect of glucosamine sulfate in two recent long-term trials using conventional standing antero-posterior radiographs, possibly due to the mild-to-moderate patient characteristics. (C) 2003 OsteoArthritis Research Society International. Published by Elsevier Ltd. All rights reserved.