This is a prospective natural history study on patients with clinically defined sIBM. Participants will be assessed every 6 months over two years (five visits total). We will include 150 participants, enrolled across 13 sites, with sporadic IBM, diagnosed according to established criteria.

This is a prospective natural history study on patients with clinically defined sIBM. Participants will be assessed every 6 months over two years (five visits total). We will include 150 participants with sporadic IBM, diagnosed according to established criteria. Participants will require an routine serum sample via blood draw to evaluate for NT5c1A antibody status. This testing will be performed at Washington University School of Medicine in the Neuromuscular Laboratory. Investigators will be blinded to antibody status. A subset of participants (40) will undergo a muscle biopsy at the Baseline visit.

Aim 1. To determine for the first time whether NT5c1A antibodies mediate disease progression over a two-year interval in patients with sIBM.

We will perform a prospective, non-interventional, observational study on patients with sIBM with follow up and evaluations every 6 months over a two-year time frame. Primary analyses are: 1) the rates of disease progression and severity as measured by rates of decline in IBM Functional Rating Scale (IBMFRS) score and Timed Get Up and Go (TUG); 2) the presence or absence of serum antibodies to NT5c1A; and 3) the presence and frequency of variant T-cells in the serum and skeletal muscle.

Aim 2. To perform a detailed morphological, histochemical and immunohistochemical analysis of fresh muscle biopsy specimens obtained from a subset of patients with sIBM.

Aim 3. To characterize the distribution of "immunosenescent" lymphocytes in circulating blood from patients with sIBM.

Aim 4. To quantify the decline in the respiratory function of sIBM patients.

Inclusion Criteria:

1. Ages 40 years and older
2. Fulfills ENMC 2011 criteria of clinically definite or probable sporadic Inclusion body myositis (sIBM)
3. Disease onset is within the past 10 years of the time of Baseline visit
4. Able to participate and comply with study related procedures
5. Able to provide written consent

Exclusion Criteria:

1. Current or very recent use (within last 6 months of the Baseline visit) of immunomodulation or immunosuppression therapy.
2. Current or very recent use (within last 90 days of the Baseline visit) of an investigational medication or therapy.
3. Co-existing significant medical or surgical conditions that, in the opinion of the investigator, will influence study participation or alter natural history.

Paticipants age 40 and older with sporadic IBM, diagnosed according to established criteria.