Purpose: To determine the relationship between changes in the extracellular matrix (ECM) and T 1ρ and T 2 values in vivo. The ECM is composed of proteoglycan (PG), collagen, and water. It has been unclear which of the ECM constituents affects T 1ρ and T 2 mapping in living human cartilage. Materials and Methods: Sagittal T 1ρ and T 2 maps were preoperatively obtained from 20 knee osteoarthritis patients. Osteochondral samples harvested from the resected tibial plateaus during total knee arthroplasty were consistent with the MRIs of the patients' knees. Parameters that included histological grading of cartilage degeneration, glycosaminoglycan (GAG) content (which constitutes PG), presence of collagen anisotropy and water content were evaluated along with T 1ρ and T 2 values, and statistical analysis was performed using multiple regression analysis. Results: T 1ρ and T 2 values were significantly correlated with the degree of cartilage degeneration (β = 0.397 and 0.357, respectively) and the GAG content (β = -0.340 and -0.244, respectively). Conclusion: The present study demonstrated that T 1ρ and T 2 values reflect the GAG content of the cartilage and can indicate cartilage degeneration in vivo. Use of these parameters can facilitate the noninvasive diagnosis and evaluation of cartilage degeneration. © 2011 Wiley Periodicals, Inc.