The CD4⁺ T-cell pool in HIV-infected patients is in a constant state of flux as CD4⁺ T cells are infected and destroyed by HIV and new cells take their place. To study T-cell survival, we adoptively transferred peripheral blood lymphocytes transduced with the neomycin phosphotransferase gene between syngeneic twin pairs discordant for HIV infection. A stable fraction of marked CD4⁺ T cells persisted in the circulation for four to eighteen weeks after transfer in all patients. After this time there was a precipitous decline in marked cells in three of the patients. At approximately six months, marked cells were in lymphoid tissues in proportions comparable to those found in peripheral blood. In two patients, the proportion of total signal for the transgene (found by PCR analysis) in the CD4/CD45RA⁺ T-cell population relative to the CD4/CD45RO⁺ population increased in the weeks after cell infusion. These findings indicate that genetically-marked CD4⁺ T cells persist in vivo for weeks to months and that the CD4⁺ T-cell pool in adults is maintained mostly by the division of mature T cells rather than by differentiation of prethymic stem cells. Thus, after elements of the T-cell repertoire are lost through HIV infection, they may be difficult to replace.