In dengue acute cases (Groups A–D), the anti-chikungunya virus IgM ELISA test showed an overall cross-reactivity of 31.6% (30/95), independently of the infecting serotype, with a higher cross-reactivity observed for DENV-4 cases (Group D; 50.0%), followed by DENV-2 (Group B; 33.3%). This evidence concerns the gene CD40LG and dengue disease.