PURPOSE: To evaluate the influence of skeletal maturation on sodium ((23) Na) MRI relaxation parameters and the accuracy of tissue sodium concentration (TSC) quantification in human knee cartilage. METHODS: Twelve pediatric knee specimens were imaged with whole-body 10.5 T MRI using a density-adapted 3D radial projection sequence to evaluate (23) Na parameters: B(1) (+) , T(1) , biexponential T2* , and TSC. Water, collagen, and sulfated glycosaminoglycan (sGAG) content were calculated from osteochondral biopsies. The TSC was corrected for B(1) (+) , relaxation, and water content. The literature-based TSC (TSC(LB) ) used previously published values for corrections, whereas the specimen-specific TSC (TSC(SP) ) used measurements from individual specimens. (23) Na parameters were evaluated in eight cartilage compartments segmented on proton images. Associations between (23) Na parameters, TSC(LB)  - TSC(SP) difference, biochemical content, and age were determined. RESULTS: From birth to 12 years, cartilage water content decreased by 18%; collagen increased by 59%; and sGAG decreased by 36% (all R(2)   ≥  0.557). The short T2* ( T2*S ) decreased by 72%, and the signal fraction relaxing with T2*S ( fT2*S ) increased by 55% during the first 5 years but remained relatively stable after that. TSC(SP) was significantly correlated with sGAG content from biopsies (R(2)  = 0.739). Depending on age, TSC(LB) showed higher or lower values than TSC(SP) . The TSC(LB)  - TSC(SP) difference was significantly correlated with T2*S (R(2)  = 0.850), fT2*S (R(2)  = 0.651), and water content (R(2)  = 0.738). CONCLUSION: TSC and relaxation parameters measured with (23) Na MRI provide noninvasive information about changes in sGAG content and collagen matrix during cartilage maturation. Cartilage TSC quantification assuming fixed relaxation may be feasible in children older than 5 years.