Retroviruses display a strong selective pressure to maintain the dimeric nature of their genomic RNAs, suggesting that dimerization is essential for viral replication. Recently, we identified the cis-element required for initiation of human immunodeficiency virus type I (HIV-I) RNA dimerization in vitro. The dimerization initiation site (DIS) is a hairpin structure containing a self-complementary sequence in the loop. We proposed that dimerization is initiated by a loop-loop kissing interaction involving the self-complementary sequence present in each monomer. We tested the ability of sense and antisense oligonucleotides targeted against the DIS to interfere with a preformed viral RNA dimer. Self-dimerization and inhibition properties of the tested oligonucleotides are dictated by the nature of the loop. An RNA loop is absolutely required in the case of sense oligonucleotides, whereas the nature and the sequence of the stem is not important. They form reversible loop-loop interactions and act as competitive inhibitors. Antisense oligonucleotides are less efficient in self-dimerization and are more potent inhibitors than sense oligonucleotides. They are less sensitive to the nature of the loop than the antisense oligonucleotides. Antisense hairpins with either RNA or DNA stems are able to form highly stable and irreversible complexes with viral RNA, resulting from complete extension of base pairing initiated by loop-loop interaction.