Dendritic cells (DC) are a heterogeneous group of uniquely well-equipped bone marrow-derived antigen-presenting cells. They circulate in blood as precursor cells (preDC). In humans, two blood-borne subtypes of preDC can be distinguished by their differential expression of CD11c (CD11c+ preDC; monocytoid DC) and CD123 (CD123+ preDC; plasmacytoid DC). We studied the incidence of monocytoid and plasmacytoid DC in peripheral blood samples from 39 children of various ages (0.4-16.8 years) by flow cytometry, and found a significant negative correlation between the number of plasmacytoid DC and age (r = 0.421, P = 0.012). Monocytoid DC counts did not change significantly with age. Similarly, we analysed DC subsets in 19 children with cancer at the time of diagnosis prior to initiation of any myelosuppressive or antiproliferative treatment and compared the results with those obtained from gender- and age-matched control children. Patients with cancer had significantly less circulating monocytoid DC than controls (medians 13.2 versus 21.4 cells/ micro l, respectively, P = 0.042) at diagnosis, whereas absolute plasmacytoid DC counts did not differ significantly between the study groups. However, clinical outcome of the children with cancer (2.9-5 years follow-up after diagnosis) correlated with plasmacytoid DC count. Children with high plasmacytoid DC counts at diagnosis (above median) survived significantly worse (6/10 deceased) than those with low counts (1/9 deceased) (P = 0.034). Thus, circulating plasmacytoid DC counts are related to age during childhood, and development of cancer is associated with low number of monocytoid DC. A low circulating plasmacytoid DC count at diagnosis was a good prognostic sign.