It was previously reported that deletions introduced into the 3'-untranslated region (3'-UTR) of dengue type 4 (DEN 4) virus (Men, R., Bray, M., Clark, D., Chanock, R.M., Lai, C.J., 1996. DEN 4 virus mutants containing deletions in the 3'-noncoding region of the RNA genome: analysis of growth restriction in cell culture and altered viremia pattern and immunogenicity in Rhesus monkeys. J. Virol. 70, 3930-3937), tick-borne encephalitis (TBE) virus (Mandl, C.W., Holzmann, H., Meixner, T., Rauscher, S., Stadler, P.F., Allison, S.L. , Heinz, F.X., 1998. Spontaneous and engineered deletions in the 3'-noncoding region of TBE virus: construction of highly attenuated mutants of a flavivirus. J. Virol. 72, 2132-2140) and subgenomic replicons of Kunjin virus (Khromykh, A.A., Westaway, E.G., 1997. Subgenomic replicons of the flavivirus Kunjin: construction and applications. J. Virol. 71, 1497-1505) altered the infectivity of the mutants and reduced the efficiency of RNA replication. Here, these deletions were superimposed onto the models of secondary structure we constructed previously and the folding of the modified 3'-UTR sequences was simulated. The analysis showed that most of the deletions disrupted or reshaped conserved elements of secondary structure and that the biological effects of these deletions are likely to represent structural rearrangements in the 3'-UTR, rather than the loss of sequence motifs. The analysis also suggested that the overall structural integrity of the flaviviral 3'-UTR is essential for optimal performance of its promotor function, although two distinct parts can be defined: the most 3'-terminal structures and sequences which may be critical for the initiation of minus-strand RNA synthesis, and more proximal structures and sequences that possibly function as enhancers of viral RNA replication. The functional significance of certain structural elements and their possible effect on the efficiency of viral replication in different cells are also discussed.