Dendritic cells (DCs) are key antigen-presenting cells with a potential role in tumor vaccines. We investigated the hypothesis that early reconstitution of DCs after allogeneic hematopoietic stem cell transplantation (SCT) improves survival. We also correlated DC reconstitution with complications of relapse and acute graft-versus-host disease (aGVHD). Fifty patients underwent transplantation between February 2000 and March 2003, with a median follow-up of 501 days (range, 136-1263 days). Most (92%) received blood stem cells, and the remainder received bone marrow from HLA-matched sibling donors for predominantly high-risk hematologic malignancies. Around the time of engraftment, peripheral blood underwent flow cytometry analysis for DCs, and the cells were divided as DC1 and DC2. Using Kaplan-Meier analysis, patients with lower DC counts (< 4.97 cells/μL) were found to have significantly worse survival (P = .002), increased incidence of relapse (P = .002), higher incidence of aGVHD onset (P = .0005), and a composite end point of relapse or death (P = .0017). A Cox proportional hazards multivariate model adjusted for important covariates confirmed that low DC count is independently associated with death (hazard ratio [HR], 3.8; P = .02), time to relapse (HR, 11.6; P = .001), and aGVHD (HR, 3.3; P = .04). Sensitivity and specificity rates for low DC count in predicting death or relapse are 73% and 75%, respectively. Low numbers of circulating DCs significantly increase the risk for relapse and acute GVHD and predict death after allogeneic SCT. (Blood. 2004;103:4330-4335)