Reconstitution of cellular immunity by 3 months after hematopoietic stem cell transplantation (HSCT) is a critical determinant of the long-term success of the transplantation. We analyzed the factors affecting recovery of cytomegalovirus (CMV)–specific CD4⁺ and CD8⁺ function at 3 months after HSCT by univariate and multivariable analyses including source of stem cells (bone marrow vs peripheral blood stem cells [PBSCs]), age, sex, graft-versus-host disease (GVHD), steroid use, conditioning regimens, ganciclovir use, HLA matching, circulating CMV antigenemia, absolute CD4⁺ and CD8⁺ counts, and donor CMV serology. High-dose steroids and CD4⁺ count less than 100 × 10⁹/L were significant predictors of impaired CD4⁺ functional recovery in the multivariable analysis. High-dose steroids, bone marrow as a source of stem cells, and CD8⁺ count less than 50 × 10⁹/L were associated with impaired CD8⁺ function in the multivariable analysis. Steroids were found to impair both CD4⁺ and CD8⁺ function in a dose-dependent manner. In the absence of high-dose steroids, low-level subclinical CMV antigenemia was found to stimulate both CD4⁺ and CD8⁺ functional recovery in recipients of ganciclovir prophylaxis. There was no difference in immune reconstitution between those who received prophylactic ganciclovir versus antigenemia-guided pre-emptive therapy. Thus, absolute CD4⁺ and CD8⁺ counts less than 100 × 10⁹/L and 50 × 10⁹/L, respectively; bone marrow as the source of stem cells; and high-dose steroid use all predict delayed recovery of functional T-cell immunity at 3 months after transplantation. Subclinical CMV reactivation while on ganciclovir appears to be a potent stimulator of T-cell function. These findings have implications for vaccination and adoptive-immunotherapy strategies in this population.