We evaluated the diagnostic performance of the rGroEL1–524 IgM-ELISA using leptospirosis paired-sera derived from northeastern Thailand, i.e., Loei, Nakhon Ratchasima, Sakol Nakhon, and controls from non-endemic Bangkok and other febrile illnesses, compared to MAT and culture methods. The gene discussed is CD40LG; the disease is leptospirosis.