We examined the recovery of circulating monocytoid (Lin- CD11+ HLA-DR+) and plasmacytoid (Lin- CD123+ HLA-DR+) precursor (pre) dendritic cell (DC) subsets after allogeneic stem cell transplantation (SCT) in 39 children, using age-matched healthy children as controls. The frequencies of DCs in peripheral blood samples were determined by flow cytometry. The initial recovery of DC occurred simultaneously with myeloid engraftment. However, with time, DC subset values declined, being very low 40-50 days after SCT. Low monocytoid and plasmacytoid DC values were associated significantly with the development of severe acute graft-versus-host disease (aGVHD) (P=0.042 and 0.017, respectively). Plasmacytoid DC values were lower than in the age-matched controls for the entire follow-up period (range 102-2569 days), although, with time, values approached normal levels. Normal monocytoid DC numbers were observed within 300-400 days post SCT. The severity of chronic GVHD did not correlate with quantitative recovery of DC. We conclude that in pediatric SCT, initial recovery of DC production is concurrent with that of myelopoiesis, yet with time, DC subset values decline and low counts are associated with severe aGVHD. Monocytoid DC numbers approach normal levels within a year of SCT, but plasmacytoid DC counts recover very slowly.