To investigate the clinical characteristics, laboratory features, and treatment outcomes of Thai patients compared between those with necrotizing autoimmune myopathy (NAM) and those with other idiopathic inflammatory myopathies (IIMs) or non-NAM. This multicenter case-control study included patients aged ≥ 18 years who were diagnosed with IIMs by muscle pathology, and who had relevant clinical and laboratory data, including muscle enzymes, from at least 3 follow-up visits during a 1-year period. Baseline clinical and laboratory data were recorded. Serum myositis-specific autoantibodies (MSAs) were obtained on the date of recruitment. Of the 70 included patients, 67% had NAM, and 33% had non-NAM. The mean age of patients was 50.5 ± 15.9 years, 67% were female, and the median duration of symptoms was 2 months (IQR, 1-4). History of cancer was significantly higher in non-NAM (21.7% vs. 2.1%, p = 0.01). Gottron's papules were significantly more prevalent in non-NAM (21.7% vs. 4.3%, p = 0.04). Non-NAM had a higher prevalence of anti-Mi-2a (17.4% vs. 2.1%, p = 0.04) and Mi-2b (17.4% vs. 0.0%, p = 0.01); however, the presence of other MSAs, including anti-HMGCR and anti-SRP, was similar between groups. Improvement in motor power and treatment intensification with glucocorticoid and/or immunosuppressive agents 3 times throughout the follow-up period was similar between groups (NAM 46.8% vs. non-NAM 34.8%, p = 0.34). NAM is indistinguishable from non-NAM by clinical manifestations, serology, or laboratory findings, except that pathognomonic skin sign of Gottron's papules and anti-Mi2 are suggestive of dermatomyositis. The integration of clinical, serological, and pathological data is essential for making a diagnosis of NAM.Key Points• NAM is indistinguishable from non-NAM by clinical manifestations, serology, or laboratory findings.• The integration of clinical, serological, and pathological data is essential for making a diagnosis of NAM.