Objective: To determine the value of quantitative MRI to provide imaging biomarkers for disease in 20 different upper and lower leg muscles of myotonic dystrophy type 1 (DM1) patients. Methods: We acquired images covering these muscles in 33 genetically and clinically well-characterized DM1 patients and 10 unaffected controls. MR images were recorded with a Dixon method to determine muscle fat fraction, muscle volume and contractile muscle volume, and a multi-spin echo sequence to determine T2 water relaxation time (T2water), reflecting putative oedema. Results: Muscles in DM1 patients had higher fat fractions than muscles of controls (15.6%±11.1% vs. 3.7%±1.5%). Also, patients had smaller muscle volumes (902±232 cm3 vs. 1097±251 cm3), contractile muscle volumes (779±247 cm3 vs. 1054±246 cm3), and increased T2water (33.4±1.0 ms vs. 31.9±0.6 ms), indicating atrophy and oedema, respectively. Lower leg muscles were affected most frequently, especially the gastrocnemius medialis and soleus. Distribution of fat content per muscle indicated gradual fat infiltration in DM1. Between-patient variation in fat fraction was explained by age (~45%), and another ~14% by estimated progenitor CTG repeat length (r2 = 0.485) and somatic instability (r2 = 0.590). Fat fraction correlated with the six-minute walk test (r = -0.553) and muscular impairment rating scale (r = 0.537), and revealed subclinical muscle involvement. Conclusion: This cross-sectional quantitative MRI study of 20 different lower extremity muscles in DM1 patients revealed abnormal values for muscle fat fraction, volume and T2water, which therefore may serve as objective biomarkers to assess disease state of skeletal muscles in these patients.