Increasing evidence suggests that alcohol abuse may be linked to adverse immunomodulatory effects on immune responses. Our study was undertaken to clarify the immunological consequences of chronic and acute alcohol exposure on differentiation and maturation of human dendritic cells (DCs). Using immunochemical and cytofluorimetric analysis we determined the phenotype and functions of monocyte-derived DCs from alcoholics and healthy subjects and analyzed their ability to respond to lipopolysaccharide (LPS) in the presence or absence of ethanol (EtOH) exposure. Our results showed that alcoholics' monocytes differentiated to immature DCs with altered phenotype and functions (alc-iDCs). Alc-iDCs showed fewer CD1a+ cells, weaker CD86 expression and higher HLA-DR expression associated with lower endocytosis and allostimulatory functions than iDCs from healthy subjects (control-iDCs). Despite these impairments, alc-iDCs produced TNF-α and IL-6 in large amounts. LPS stimulation failed to induce full phenotypical and functional alc-iDC maturation. In vitro acute EtOH exposure also prevented alc-iDCs and control-iDCs from maturing in response to LPS. T-cell priming experiments showed that EtOH treatment prevented LPS-stimulated control-iDCs from priming and polarizing naïve allogeneic T cells into Th1 cells, thus favouring a predominant Th2 environment. Collectively, our results provide evidence that chronic and acute alcohol exposure prevents DCs from differentiating and maturing in response to a microbial stimulus.