Background: Our objective was to define the pattern and severity of muscle damage in immune-mediated necrotizing myopathy (IMNM) and its relationship with clinical and serological features. Methods: IMNM patients with a whole-body MRI (n=42) were included and compared to sporadic inclusion-body myositis (s-IBM) patients (n=60). Fat replacement was estimated using the Mercuri score in 55 muscles. Overall lesion load was defined as the sum of all abnormal Mercuri scores (reported in % maximal score) and lesion load quotient was defined as the overall lesion load divided by disease duration. Linear relationships between variables were assessed and multidimensional analysis was performed to define homogenous groups of patients. Results: IMNM patients were aged 48.1±15.8 years and had a disease duration of 9.8±8.1 years. Most severely affected muscle groups were located in the pelvifemoral and lumbar region. Unsupervised analysis showed two subgroups of patients: one with mild lesion load (15±10%, n=32/42) and another with severe lesion load (60±10%, n=10/42: p<0.001) associated with a mean disease duration of 6.8±6.0 years and 19.5±5.7 years, respectively (p<0.0001). Correlational studies confirmed that disease duration was the most important predictor of muscle damage. Multivariate analyses demonstrated a more severe involvement in select muscle groups in females and seropositive patients. No difference was found in overall lesion load quotient of IMNM compared to IBM (p=0.07) but with a distinct muscle pattern. Conclusion: IMNM is associated with severe axial and pelvifemoral muscle damage. Disease duration is an important predictor of muscle damage. IMNM and s-IBM patients have a comparable damage burden.