Safety and Efficacy of Avonex in Subjects With Chronic Inflammatory Demyelinating Polyradiculoneuropathy (CIDP)

Inclusion Criteria: * Written informed consent prior to any testing under this protocol * Must be between 18 and 75 years of age * Have a diagnosis of CIDP as determined by a board-certified or board-eligible neurologist, or equivalent, for at least 6 months, including fulfilling modified INCAT neurophysiological criteria for CIDP,CIDP motor deficit responsive to IVIg and alternative EP data that justifies subject inclusion, and/or supportive pathologic or laboratory data that supports the diagnosis of CIDP * Documentation in the medical record prior to screening that the CIDP had been associated with loss of muscle strength, such that the MRC sum score was less than or equal to 58. * Documentation in the medical record that the patient benefited fom IVIg treatment (patient had a 2-point change or equivalent in 60-point MRC sum score) * Tested for IgM monoclonal gammopathy and found to have tested negative for IgM monoclonal gammopathy or if positive for IgM monoclonal gammopathy, then are MAG antibody-negative and proven to be IVI g responsive per protocol. * Must have been clinically stable while on a constant regimen of IVIg (every 2-weeks, 3-weeks, 4-weeks or 5-weeks) in the 3 months prior to screening. Exclusion Criteria\*: * Associated systemic disorder that might cause neuropathy. * History of, or abnormal laboratory results indicative of any significant major disease or known drug hypersensitivity that, in the opinion of the investigator, would preclude the administration if IFN-beta or participation in this study. * Subjects with diabetes mellitus will not be eligible, unless they satisfy both of the following requirements: a) their diabetes is well-controlled, with no retinopathy or nephropathy, having been identified during the ongoing care of their diabetes; and b) they have a normal sensory nerve action potential (SNAP) amplitude recorded in the sural nerve on at least one side of the body identified during electrophysiology (EP) testing documented in their medical record. * Abnormal screening or baseline blood tests that the investigator deems clinically significant * History of a seizure disorder prior to baseline (Visit 1, Week 0). * History of suicidal ideation within 3 months prior to Baseline Visit (Week 0) or an episode of severe depression within 3 months prior to Baseline Visit (Week 0). * Pure motor syndrome fulfilling criteria for multifocal motor neuropathy with conduction block. * Pure sensory CIDP, or any other variant of CIDP without motor involvement * Serious local infection or systemic infection within the 6 months prior to Screening. * Use of IFN-beta at any time, use of plasma exchange, plasmapheresis, or any other immunosuppressant (with the exception of oral or non-systemic corticosteroids) within 6 months prior to Screening. * History of intolerance to acetaminophen (paracetamol), ibuprofen, naproxen, and/or aspirin that would preclude use of at least one of these during the study. * For female subjects, unless postmenopausal or surgically sterile, unwillingness to practice effective contraception, as defined by the Investigator, during the study. * Female subjects considering becoming pregnant while in the study * Female subjects who are currently pregnant or breast-feeding. * This list is not exhaustive and there may be additional exclusions