The objectives of the study were to investigate the number of myeloid dendritic cells (mDC) and plasmacytoid dendritic cells (pDC) present in peripheral blood mononuclear cells (PBMC) from severe aplastic anemia (SAA) patients before and after intensive immunosuppressive therapy (IST) and to assess the expression of co-stimulatory molecules (CD80, CD86, and CD40) expressed by dendritic cells (DC) from SAA patients. The quantities of mDC and pDC and ratios of mDC to pDC in PBMC were measured in 38 SAA patients at active phase, 19 patients at recovery phase, and 17 normal controls. The surface expression of CD80, CD86, and CD40 on DCs and B lymphocytes was analyzed in 16 SAA patients and 15 normal controls. The percentages of mDC and the ratio of mDC:pDC of SAA patients at active phase increased compared to that of healthy controls [0.65% (range 0.10-2.19%) vs. 0.40% (range 0.11-1.54%), 2.64% (range 1.07-4.33%) vs. 1.56% (range 0.89-2.27), respectively (P < 0.05)]. The percentages of mDCs in recovered SAA patients decreased to 0.43% (range 0.06-0.80), and the ratio of mDC:pDC decreased to 1.78% (range 0.49-3.07). The percentages of mDC and pDC in 10 SAA patients were 0.87% (range 0.10-1.85) and 0.35% (range 0.05-0.65) before IST, which decreased to 0.24% (range 0.06-0.52) and 0.14% (range 0.01-0.28) after IST (P < 0.05). The percentages of CD86 expression on DC of SAA patients increased compared to that of healthy controls [29.84% (range 20.28-39.40) vs. 11.97% (range 0.02-24.15), respectively (P < 0.05)]. The number of mDCs increased in SAA patients, which was associated with stage of disease. The increased number of mDCs and the high expression of costimulatory molecules (CD86) on these DCs may contribute to abnormal activation of T lymphocytes in these patients and subsequent immune system-mediated bone marrow failure.