Zika is an emerging mosquito-borne illness known to circulate in Africa, the Americas and the Pacific, in areas where the dengue viruses (DENV) are endemic. Zika virus (ZIKV) is a single-stranded RNA virus of the Spondweni group in the Flavivirus genus. The flaviviruses display a high degree of sequence homology, and ZIKV is closely related to DENV with approximately 43% amino acid identity across the viral polyprotein as well as the ectodomain of the envelope protein (E). The flavivirus E glycoprotein is the principal antigen in induction of neutralising antibodies that may contribute to protection and which are a useful marker for vaccine-induced immunity. We assessed serum neutralising antibody (Nab) profiles against ZIKV strain MR 766; DENV-1-4; Japanese encephalitis virus, vaccine strain IMOJEV (JEV); and yellow fever virus, vaccine strain 17D (YFV) in 3 patient groups: (1) suspected and confirmed acute ZIKV infection; (2) acute and late convalescent dengue infection; and (3) flavivirus vaccine recipients. Nab titre was determined using a focus reduction neutralisation test (FRNT), with 50%, 75% and 90% endpoints calculated. Sera from ZIKV cases showed high ZIKV and DENV titres suggesting previous DENV infection whereas other cases showed high ZIKV and low or no DENV titres suggestive of primary monotypic ZIKV infection. Some ZIKV cases also had high YFV titres consistent with previous YFV vaccination. Sera collected during acute phase monotypic DENV infection showed high magnitude ZIKV and DENV titres and cross-reactivity with YFV, in the absence of previous vaccination. Convalescent DENV sera displayed high titres to homologous and heterologous serotypes, and low or no ZIKV titres in most patients; one patient showed FRNT-90 ZIKV titres suggesting previous ZIKV infection. Comparison of anti-ZIKV Nab profiles against strains MR 766 (Rhesus monkey, 1947), PRVABC59 (human, 2015) and FP (human, 2013) is being undertaken. Results so far show that Nab profiles among patients are associated with stage of infection, vaccine status and likely previous flavivirus exposure.