Viremia is significantly lower in HIV-2 than in HIV-1 infection, irrespective of disease stage. Nevertheless, the comparable proviral DNA burdens observed for these two infections indicate similar numbers of infected cells. Here we investigated this apparent paradox by assessing cell-associated viral replication. We found that untreated HIV-1-positive (HIV-1(+)) and HIV-2(+) individuals, matched for CD4 T cell depletion, exhibited similar gag mRNA levels, indicating that significant viral transcription is occurring in untreated HIV-2(+) patients, despite the reduced viremia (undetectable to 2.6 × 10(4) RNA copies/ml). However, tat mRNA transcripts were observed at significantly lower levels in HIV-2(+) patients, suggesting that the rate of de novo infection is decreased in these patients. Our data also reveal a direct relationship of gag and tat transcripts with CD4 and CD8 T cell activation, respectively. Antiretroviral therapy (ART)-treated HIV-2(+) patients showed persistent viral replication, irrespective of plasma viremia, possibly contributing to the emergence of drug resistance mutations, persistent hyperimmune activation, and poor CD4 T cell recovery that we observed with these individuals. In conclusion, we provide here evidence of significant ongoing viral replication in HIV-2(+) patients, further emphasizing the dichotomy between amount of plasma virus and cell-associated viral burden and stressing the need for antiretroviral trials and the definition of therapeutic guidelines for HIV-2 infection.