Many of the common mimics of idiopathic inflammatory myopathies (IIMs) can be rapidly excluded at initial evaluation with appropriate clinical and investigational tools. More fastidious conditions may be missed on initial evaluation. We aimed to study the clinico-investigational profile and final diagnosis of patients who were initially diagnosed and treated as IIMs and received an alternate diagnosis in follow-up. We screened the medical records of patients with suspected IIM seen in our neuromuscular clinic or wards between January 2019 to December 2023. We included patients who were diagnosed as IIM (in this or an alternate centre) based on clinical, serological or pathological correlation and given immunotherapy, and whose diagnoses were altered in follow up. We excluded patients who were reclassified as infectious or post-viral myositis and Sporadic inclusion body myositis (s-IBM) from this analysis. The data was extracted with a structured proforma, and descriptive analysis was performed. We reviewed 112 records of suspected IIMs. After excluding 35 patients with confirmed IIMs, 9 s-IBM, and 57 with an alternate diagnosis after initial evaluation, we identified 11 patients who fulfilled our inclusion criteria. The male:female ratio was 6:7, and age of onset ranged from 16-64 years. The final diagnosis of IIMs mimics were genetic muscle diseases in 6, Sporadic late onset nemaline myopathy (SLONM) in 2 and Sarcoid myopathy, Scleromyxedema myopathy and Corticosteroid myopathy in one each. The genetic diseases were dysferlinopathy in 2, facioscapulohumeral muscular dystrophy in 1, GNE myopathy in 1, and GGPS1-associated muscular dystrophy in 1. All patients had a limb-girdle weakness at onset except disto-proximal weakness in GNE myopathy, 2 had early dysphagia. Symptom duration at initiation of immunotherapy was <1 year in 7 patients, <2 years in 3 and 7 years for 1. The latter 4 had a rapid decline of functional status over 3 to 8 months prior to immunotherapy initiation. 8 patients had elevated creatine kinase levels, 4 had myositis-associated or other antibody positivity, 3 had M-band in serum protein electrophoresis and 3 had STIR hyperintensities in muscle MRI. None had myositis-specific antibodies. Muscle biopsy consistent with inflammatory myopathy was reported in patients with FSHD and GNE myopathy while co-existent inflammation was reported in sarcoid myopathy and scleromyxedema myopathy. All patients received immunotherapy with duration of therapy ranging from 3 months to 7 years. The median time to final diagnosis was 27 (range 5-180) months. The most common cause of misdiagnosis was acute or subacute presentation (in 6) or acute worsening (in 4). Muscle biopsy contributed to misdiagnosis in 2 patients while it clinched the final diagnosis in 7 of 11 patients. Genetic studies were diagnostic in 4 patients. Poor immunotherapy responsiveness noted in 7, confirmatory phenotype on follow-up in 3 (FSHD, GNE myopathy, corticosteroid myopathy) and abnormal ancillary tests in 3 (sarcoid myopathy, SLONM, scleromyxedema myopathy) were the main indications for re-evaluation. Careful phenotyping and judicious application of serological, radiological and pathological information could help in the diagnosis of IIM mimics. Poor immunotherapy responsiveness and absence of myositis-specific antibodies warrant evaluation. [ABSTRACT FROM AUTHOR]