The first 111 nt from the 5′ end of human immunodeficiency virus type 1 (HIV‐1) mRNAs are shown to have a strong inhibitory effect on the translation of mRNA in in vitro translation extracts as well as in Xenopus oocytes. Mutations in the sequence of the 5′ untranslated region (UTR) designed to disrupt predicted secondary structure of this region relieve the inhibition. Inhibition is restored by mutations that reconstruct the predicted secondary structure. The accessibility of the 5′‐terminal cap structure was also found to be increased by some of these mutations. We conclude that secondary structure in the 5′ UTR of HIV‐1 mRNAs and resultant inaccessibility of the cap structure is responsible for the inhibition of translation. The implications of these findings for the understanding of the life cycle of HIV‐1 are discussed.