This study determines the presence and extent of muscle changes in 31 myotonic dystrophy type 2 (DM2) patients detected by muscle ultrasound. Results were compared to 31 adult-onset myotonic dystrophy type 1 patients (DM1) and healthy controls. Furthermore, we tested the hypothesis that structural muscle changes correlate with age, quantitative muscle force and serum creatine kinase in both disorders. In DM2 all seven examined muscles (right masseter muscle, right and left biceps brachii, right and left forearm flexors, right rectus femoris, and left tibialis anterior muscle) showed increased mean echo intensities (p ≤ 0.001). Atrophy of the masseter muscle and rectus femoris were both found in 23% of DM2 patients. Muscle thickness was significantly more decreased in the elbow flexors in DM2 compared to DM1. Echo intensity sum score correlated positively with age in DM2 (r=0.57, p=0.001) and negatively with muscle force (r=0.36, p=0.048). We conclude that all tested muscles are affected and structurally abnormal in DM2 patients. Proximal arm muscles are more affected in DM2 compared to DM1, which corresponds to clinical findings.