Dendritic cells (DCs) are functionally abnormal in systemic lupus erythematosus (SLE). However, previous studies have involved in-vitro cytokine-induced DCs. In this investigation, directly isolated circulating plasmacytoid DCs (pDCs) and myeloid DCs (mDCs) in SLE were studied. Blood dendritic cell antigen (BDCA)-4 and BDCA-1 magnetic isolation kits were used to isolate blood pDCs and mDCs from 30 SLE patients and 36 controls. Their number and surface markers, and their relationship with lupus disease activity were evaluated. The percentage of pDCs per peripheral blood mononuclear cells was higher in SLE (0.33+/-0.14) than in controls (0.16+/-0.09, P<0.01), but that of mDCs was lower in SLE (0.43+/-0.14) than in controls (0.63+/-0.32; P<0.01). In controls, both pDCs and mDCs expressed high levels of MHC-II, however, the expression of CD86, CD83 and CCR7 on pDCs were significantly lower than that on mDCs (all P<0.05). mDCs from patients with SLE, particularly those with active disease, expressed lower CD83 than controls. In health, circulating mDCs may be more efficient than pDCs in stimulating T cells. In SLE, the increased number of circulating pDCs supports a pathogenic role for these cells, and the decreased mDC number and CD83 expression may explain the susceptibility to infections in these patients.