Objective: To evaluate serum matrix metalloproteinase (MMP)-3 levels as a prognostic marker for the progression of cartilage damage in patients with knee osteoarthritis (KOA). Methods: Fifty-six patients, aged 40 to 80 years (62.59 ± 10.11 years) who met the ACR criteria for KOA, were included in a one-year observational prospective clinical study. Complete baseline and follow-up data were collected from 50 out of 56 patients. X-ray and magnetic-resonance images were carried out at baseline and after 12 months. They were evaluated according to the Kellgren-Lawrence and Whole-Organ magnetic Resonance iMaging Score (WORMS) semi-quantitative scales, respectively. Progression of cartilage damage in the medial tibiofemoral compartment was registered at the end of the follow-up using the change in WORMS. Serum levels of MMP-3 were measured during the baseline visit, using enzyme-linked immunosorbent assay. Results: Significantly higher values of baseline MMP-3 levels were observed in patients with a registered progression of cartilage injury in the medial tibiofemoral compartment of the knee compared with patients with no progression (p = 0.005). Binary logistic regression analysis showed that levels of serum MMP-3 (ng/ml) were an independent predictor of subsequent progression of cartilage injury in the medial tibiofemoral compartment of the index knee (assessed by MRI) (OR = 1.042, CI 95%: 1.002-1.084). Receiver operating characteristic analysis was performed to delineate progressors from non-progressors. Conclusion: Serum MMP-3 levels may serve as a potential prognostic biomarker for cartilage injury in patients with KOA.