Damage to the small nerve fibers that sense pain and regulate function of internal organs results in small-fiber polyneuropathy (SFN). SFPN symptoms include unexplained chronic widespread pain (CWP) and chronic multisymptom illness (CMI) similar to Gulf War Illness. Our prior research demonstrated that SFPN is prevalent in such CWP and CMI syndromes and that it can have onset at a young age. Given these non-specific symptoms, objective testing is recommended for SFPN diagnosis. In the third year of this study Global experts participated in additional rounds of a Delphi process to determine the most reliable markers for SFPN (Case Definition). We also developed a comprehensive database of SFPN patients and well characterized controls with which to validate the Case Definition. The diagnostic tests recommended under the Case Definition were applied to Veterans and age-matched controls to look for the prevalence of SFPN.