A series of unusual folding regions (UFR) immediately 3' to the cleavage site of the outer membrane protein (OMP) and transmembrane protein (TMP) were detected in the envelope gene RNA of the human immunodeficiency virus (HIV-1, HIV-2) and simian immunodeficiency virus (SIV) by an extensive Monte Carlo simulation. These RNA secondary structures were predicted to be both highly stable and statistically significant. In the calculation, twenty-five different sequence isolates of HIV-1, three isolates of HIV-2 and eight sequences of SIV were included. Although significant sequence divergence occurs in the env coding regions of these viruses, a distinct UFR of 234-nt is consistently located ten nucleotides 3' to the cleavage site of the OMP/TMP in HIV-1, and a 216-nt UFR occurs forty-six and forty-nine nucleotides downstream from the OMP/TMP cleavage site of HIV-2 and SIV, respectively. Compensatory base changes in the helical stem regions of these conserved RNA secondary structures are identified. These results support the hypothesis that these special RNA folding regions are functionally important and suggest that the role of this sequence as the Rev response element (RRE) is mediated by secondary structure as well as primary RNA sequence.