To investigate the influence of pre-existing antibodies against tick-borne encephalitis (TBE) or yellow fever (YF) viruses on dengue virus antibody test results, we examined sera from vaccinees and from individuals with previous TBE virus infection. Distinct IgG antibody cross-reactivity was found in about 15.1% in the YF-vaccinated group and in about 9.5% in the TBE-vaccinated group. Altogether 15 out of a total of 80 samples tested (18.8%) had detectable dengue virus IgG antibody titres. The serum samples from patients with acute TBE virus infection not only had the highest anti-TBE virus antibodies but were also highly cross-reactive against dengue virus antigens. The high cross-reactivity rate of YF and TBE antibody-positive sera in dengue virus antibody assays should be taken into account in the interpretation of laboratory tests for the diagnosis of flavivirus infections and when undertaking seroepidemiological surveys.