One of the consequences of viral quasispecies dynamics is the presence, in the mutant spectrum, of minority memory genomes that reflect those variants that were dominant at an earlier phase of the same evolutionary lineage. Replicative and cellular (or anatomical) contributions to quasispecies memory were previously defined during intrahost evolution of human immunodeficiency virus type 1 (HIV-1) [Briones, C., Domingo, E., Molina-París, C., 2003. Memory in retroviral quasispecies: experimental evidence and theoretical model for human immunodeficiency virus. J. Mol. Biol. 331, 213-229.]. However, the effects of replicative memory regarding virus evolution in vivo have not been investigated. Here we document that a multidrug-resistant (MDR) HIV-1, present at memory level, determined the ensuing evolution of the virus in an infected patient. Nucleotide sequencing and detailed phylogenetic analyses of sequential viral populations and individual molecular clones evidenced that the progeny of a minority MDR genome during a treatment interruption contributed the dominant genomes when an antiretroviral treatment was restored. An extension of a mathematical model of establishment and maintenance of memory, based on quasispecies theory, supports the experimental data. Therefore a replicative memory subpopulation, not detectable in a consensus nucleotide sequence, affected decisively subsequent states of viral evolution in vivo.