The immunological profile (antinuclear antibodies, polynuclear neutrophil anticytoplasmic antibodies, and antiendomysium antibodies) was negative. Immunological tests for intraneuronal antibodies, by immunochemistry and immunoblot including anti-Hu, anti-Yo, anti-Ri, antiamphiphysin, anti-CV2, anti-Ta, and Ma1, were unremarkable. Results of Legionella, cytomegalovirus, Epstein–Barr virus, human immunodeficiency virus, and Lyme disease serologies were likewise negative and levels of vitamins B1, B6, B9, and B12 were normal. Analysis of the cerebrospinal fluid (CSF) revealed 335 leukocytes/mm3 with 99% lymphocytes, and results of T whipplei PCR targeting a 191 base pair–specific fragment of the 16S ribosomal DNA were positive on 2 different aliquots of CSF. The specificity was confirmed by direct sequencing of the amplified DNA fragment showing 100% similarity with T whipplei DNA only. Results of PCR for Herpesviridae (including herpes simplex virus 1, herpes simplex virus 2, varicella-zoster virus, cytomegalovirus, and human herpesvirus 6) were negative. Results of the T whipplei PCR assay were negative on a duodenal biopsy specimen.