Adoptive cell therapy using cytomegalovirus (CMV)-specific cytotoxic T lymphocytes (CMV-CTLs) has demonstrated efficacy posttransplant. Despite the predicted limited engraftment of CMV-CTLs derived from third-party donors, partially matched third-party donor–derived CMV-CTLs have demonstrated similar response rates to those derived from primary hematopoietic cell transplantation donors. Little is known about the mechanisms through which adoptive cellular therapies mediate durable responses. We performed a retrospective analysis of patients receiving CMV-CTLs for treatment of CMV viremia and/or disease after allogeneic transplant between September of 2009 and January of 2018. We evaluated whether response to adoptively transferred CMV-CTLs correlated with immune reconstitution (IR), using validated CD4⁺ IR milestones of 50 × 10⁶/L and 200 × 10⁶/L. In this analysis, a cohort of 104 patients received CMV-CTLs derived from a primary transplant donor (n = 25), a third-party donor (n = 76), or both (n = 3). Response to therapy did not increase the likelihood of achieving CD4⁺ IR milestones at 1 (P = .53 and P > .99) or 2 months (P = .12 and P = .33). The origin of CMV-CTLs did not impact subsequent CD4⁺ IR. CMV-CTLs appeared to interact with host immunity in mediating responses. Recipients with a baseline CD4 >50 × 10⁶/L had higher response to therapy (P = .02), improved overall survival (P < .001), and protection from CMV-related death (P = .002). Baseline endogenous immunity appears to improve CMV-related and overall survival in this cohort and can be an important marker at the initiation of therapy.