Experimental evidences on the adaptive immune response in patients with hereditary hemorragic telagiectasia (HHT) are lacking. Here, we report in 9 patients with HHT a multiple deficit involving the intracellular expression of T helper (h)1-derived cytokines [Interferon (IFN)-γ, Interleukin (IL)-2 and Tumor Necrosis Factor (TNF)-α] and of monocyte- derived TNF-α. On the other hand, percentages of Th2-derived cytokines (IL-4, IL-5 and IL-10) were normal or, in some cases, above normality. Quite interestingly, monocyte-derived IL-10 was detectable in 5 out of 9 patients in a percentage of cells comparable to controls or exceeding normal levels. Taken together, these data point out, in HHT, an ablation of Th1-responses, while Th2-type cytokines are preserved, thus exerting either a suppressive effect on Th1-cells (via IL-4 and IL-10) or an antiinflammatory response on monocyte-derived TNF-α (via IL-10). Furthermore, monocytederived IL-10 may also contribute to the antiinflammatory activity seen in HHT. According to current literature even if patients with HHT do not exhibit certain diseases, such as autoimmune diseases, cancer and abnormal responses to pathogens, the observed immune deficits need to be diagnosed and therapeutically corrected.