This is a pilot study (phase 1 clinical trial) to evaluate the safety and tolerability of phenylbutyrate in IBM. In this open label study, 10 patients with sporadic inclusion body myositis will be treated with phenylbutyrate (3 gm twice daily) for 3 months. There will be a run-in period, during which certain biomarkers will be measured at baseline and at the end of the run-in period in addition to final measurement at the end of the treatment period.

Inclusion Criteria:

• Fulfill ENMC 2011 diagnostic criteria for IBM
• Age > 18 years
• Women must be post-menopausal (no menses in >12 months) or status post hysterectomy
• Able to give informed consent

Exclusion Criteria:

• Presence of any one of the following medical conditions: chronic infection; chronic renal insufficiency; cancer other than skin cancer less than five years prior; multiple sclerosis or prior episode of central nervous system demyelination; or other chronic serious medical illnesses
• Presence of any of the following on routine blood screening: WBC<3000; Platelets < 100,000; hematocrit < 30%; BUN > 30 mg %; creatine > 1.5 mg%; liver disease with serum albumin < 3 G/DL
• Women who are pregnant or lactating
• History of non-compliance with other therapies
• Coexistence of other muscular disease
• Drug or alcohol abuse within past three months
• Known bleeding disorder
• Known liver disease
• Known congestive heart failure
• Known hypernatremia
• Inability to give informed consent