The disease dengue (DEN) is caused by four serologically related viruses termed DEN1, DEN2, DEN3 and DEN4. The structure of the ectodomain of the envelope protein has been determined previously for DEN2 and DEN3 viruses. Using NMR spectroscopic methods, we solved the solution structure of domain III (ED3), the receptor-binding domain, of the envelope protein of DEN4 virus, human strain 703-4. The structure shows that the nine amino acid changes in ED3 that separate the sylvatic and human DEN4 strains are surface exposed. Important structural differences between DEN4-rED3 and ED3 domains of DEN2, DEN3 and other flaviviruses are discussed.