Abstract There are overlapping world distributions of the mosquito-borne dengue viruses (DENVs) and water-borne bacterial disease leptospirosis which cause large numbers of human infections and fatalities. As such, early differential diagnosis is required for appropriate early leptospirosis antibiotic therapy or DENV patient supportive care, but co-infections have also been reported. From 200 paired (S1 and S2) serum samples collected from suspected DENV infected patients, 70 (35%) were confirmed as ‘on-going’ infections by demonstrating > 4-fold S1 to S2 sample increased anti-DENV IgG and/or IgM ELISA titers. Of those, 8.57% (6/70) also displayed > 4-fold increased S1 to S2 sample titers in the ‘gold standard’ Leptospira spp. microscopic agglutination test (MAT) and paraformaldehyde (cell-membrane-impermeable fixative) treated leptospires in immunofluorescence assays (IFAs) due to cross-reactions with 68-72 and 38-42 KDa outer surface membrane antigens present on all Leptospira spp. serovars tested. While DENV-1, -2 or -3 serotypes were isolated from their S1 sera: a) Leptospira spp. could not be isolated from them, b) their S1 sera were all PCR-negative using a Leptospira spp.-specific gene target, and c) their S2 sera were all negative using a commercial anti- Leptospira spp. IgM ELISA. As such, we believe this is the first report of DENVs causing false positive reactions in the ‘gold standard’ Leptospira spp. MAT, IFAs, and immunoblot assays and which needs further assessments using patients’ samples which were possibly falsely reported as ‘DENV- Leptospira spp. co-infections’ in other studies, and to identify these 68-72 and 38-42 KDa Leptospira spp. outer surface membrane antigens by proteomic analyses.