T1ρ magnetic resonance imaging (MRI) can be used to map proteoglycan (PG) loss in cartilage. Here, we used T1ρ MRI to map cartilage degradation in osteoarthritis (OA) and rheumatoid arthritis (RA). Tissue samples were obtained from five RA patients and 14 OA patients following total knee arthroplasty (TKA). Three parameters were measured: First, macroscopic grading of cartilage sample tissues was performed on a 5-grade scale (G0: normal, G1: swelling, G2: superficial fibrillation, G3: deep fibrillation, G4: subchondral bone exposure). Second, semi-quantitative values of PG were assessed by measuring the optical density of Safranin-O-stained paraffin sections that had been digitally photographed. Third, cartilage was divided into superficial and deep layers and the T1ρ values were quantified. T1ρ values of OA and RA in the superficial layers showed significant differences between groups (G0/1 and G0/2 for OA; G0/2 and G1/2 for RA). In the deep layers, T1ρ values of OA and RA also differed significantly between groups. In both the superficial and deep layers, there was a significant correlation between the mean T1ρ values and macroscopic grading (P < 0.01 for OA, P < 0.001 for RA). We found a negative correlation between the score of Safranin-O staining and T1ρ values (r = -0.61 for OA, r = -0.79 for RA). In addition, RA subjects had significantly higher T1ρ values than OA subjects of similar morphologic grade. In conclusion, T1ρ MRI is able to detect and map the early stages of cartilage degradation in OA and RA. This method is reliable and useful for the evaluation of macromolecular changes in arthritic cartilage.