Dendritic cells (DCs) are crucial in adaptive immunity because they are the only antigen-presenting cells that can present antigens to naive T lymphocytes. Plasmacytoid DCs (pDC) are also the main producers of type I Interferons in response to infection. We have shown that circulating myeloid DC (mDC) and pDC numbers are reduced in chronic as well as primary HIV infection. Data from different laboratories indicate that pDC counts, obtained by flow cytometry and rare event analysis, correlate inversely with the viral load, may be an early marker of recovery after antiretroviral treatment, and may predict better immune control of HIV replication. PDC counts may also be predictive of severe illness in dengue virus infection or of successful treatment against Mycobacterium tuberculosis . DC counts, or the “dendritogram”, may therefore become useful in the clinical assessment of different infectious diseases.