Adult patients with suspected or confirmed idiopathic inflammatory myopathy (IIM) will be recruited. Patients will be approached, consented, have baseline demographics, diagnostics and disease activity measures recorded, and blood taken. The collection of data and biological material will mirror usual clinical practice as far as possible. Subjects will ideally attend further visits at 3, 6 and 12 months to have bloods taken, outcome measures recorded and questionnaires completed.In addition, blood, muscle biopsies and imaging undertaken as part of usual care will also be collected for research purposes to measure a number of biomarkers for the assessment of diagnostic accuracy and clinical utility evaluation. As per usual practice, a muscle biopsy will be performed at baseline, and a further biopsy offered at 6 months to assess treatment response. A magnetic resonance (MR) muscle protocol will also be performed as per usual clinical practice, and a gadolinium-enhanced MR heart scan offered. Both these scans will be repeated at 6-12 months. An existing electronic database entry system will be used for data entry and capture on an anonymised basis.

Inclusion Criteria:

• All patients who had a confirmed (muscular biopsy, electromyogram, magnetic resonance imaging) or suspected clinically myositis
• Age over 18 years old
• Signature of the informed consent form for the study

Exclusion Criteria:

• Patients with disease duration >2 years
• Patients < 18 years
• Confirmed non-inflammatory myopathies
• Myositis secondary to alcohol or drug abuse
• Patients unwilling or unable to give consent
• Patients with poor or no venous access

All patients who had a confirmed or suspected myositis