Runners did not demonstrate any gross morphologic MRI changes after running a marathon. Postmarathon studies, however, revealed significantly higher T2 and T1ρ values in all articular cartilage areas of the knee (P<.01) except the lateral compartment. The T2 values recovered to baseline except in the medial femoral condyle after 3 months. Average T1ρ values increased after the marathon from 37.0 to 38.9 (P<.001) and remained increased at 3 months.