This report summarizes the recommendations of the multiple system atrophy (MSA) Working Group on Diagnostic Neuropathology Criteria for MSA that was part of an international MSA Workshop held on 26 and 27 April 2007 in Boston, MA. The workshop was supported by a grant from the National Institute of Neurological Disorders and Stroke that was intended to convene a group of international experts to revise and update criteria for the clinical and neuropathological diagnosis of MSA. The MSA Workshop recognized the glial cytoplasmic inclusions (GCIs) composed of filamentous alpha-synuclein as a defining morphological feature of MSA, and it recommends that widespread GCIs should be a criterion for the definite neuropathological diagnosis of MSA. The deliberations and recommendations of the Working Group on Diagnostic Neuropathology Criteria for MSA are summarized in this report.