PURPOSE: To investigate MRI biomarkers of muscle atrophy during cast immobilization of the lower leg. MATERIALS AND METHODS: Eighteen patients (8 male, 10 female), who had one lower leg immobilized in a cast, underwent 3.0 Tesla (T) MR imaging 5, 8, 15, 29, and 43 days after casting. Measurements were made on both lower legs of total muscle volume. Cross-sectional area (CSA), fractional water content, and T(2) were measured in tibialis anterior (TA), gastrocnemius medialis (GM) and lateralis (GL) and soleus (SOL). Fiber pennation angle was measured in GM. RESULTS: Total muscle volume decreased by 17% (P < 0.001) over the 6 weeks of immobilization. The greatest loss in CSA (mean[SD]) was seen in GM (-23.3(8.7)%), followed by SOL (-19.0(9.8)%), GL (-17.1(6.5)%), and TA (-10.7(5.9)%). Significant reductions of CSA were also detectable in the contra-lateral leg. T(2) increased in all muscles: TA 27.0(2.5) ms to 29.6(2.8) ms (P < 0.001), GM 34.6(2.9) ms to 39.8(5.4) ms (P < 0.001) and SOL 34.4 (2.9) ms to 44.9(5.9) ms (P < 0.001). Small reductions were found in fractional water content. Pennation angle decreased in the cast leg (P < 0.001). CONCLUSION: Quantitative MR imaging can detect and monitor progressive biochemical and biophysical changes in muscle during immobilization.