The complete nucleotide sequence of the genome from two yellow fever (YF) virus vaccine strains, 17DD and 17D-213, has been determined. Comparison of these sequences with those of other YF viruses including the parental virulent Asibi strain allowed the identification of 48 nucleotide sequence differences which are common to all 17D substrains. This is a significant reduction from the 67 nucleotide changes originally reported as being 17D-specific and potentially related to viral attenuation. The 48 changes are scattered throughout the genome, 26 of which are silent and 22 led to amino acid substitutions. These 22 changes are bona fide candidates to test by mutating the infectious YF cDNA to investigate their role in viral attenuation.