Allergic diseases such as asthma and rhinitis, as well the early phase of atopic dermatitis, are characterized by a Th2-skewed immune environment. Th2-type cytokines are upregulated in allergic inflammation, whereas there is downregulation of the Th1-type immune response and related cytokines, such as interferon-&#947; (IFN-&#947;). The latter is a strong inducer of indoleamine 2,3-dioxygenase-1 (IDO-1), which degrades the essential amino acid tryptophan, as part of an antiproliferative strategy of immunocompetent cells to halt the growth of infected and malignant cells, and also of T cells - an immunoregulatory intervention to avoid overactivation of the immune system. Raised serum tryptophan concentrations have been reported in patients with pollen allergy compared to healthy blood donors. Moreover, higher baseline tryptophan concentrations have been associated with a poor response to specific immunotherapy. It has been shown that the increase in tryptophan concentrations in patients with pollen allergy only exists outside the pollen season, and not during the season. Interestingly, there is only a minor alteration of the kynurenine to tryptophan ratio (Kyn/Trp, an index of tryptophan breakdown). The reason for the higher tryptophan concentrations in patients with pollen allergy outside the season remains a matter of discussion. To this regard, the specific interaction of nitric oxide (NO<b>∙</b>) with the tryptophan-degrading enzyme IDO-1 could be important, because an enhanced formation of NO<b>∙</b> has been reported in patients with asthma and allergic rhinitis. Importantly, NO<b>∙</b> suppresses the activity of the heme enzyme IDO-1, which could explain the higher tryptophan levels. Thus, inhibitors of inducible NO<b>∙</b> synthase should be reconsidered as candidates for antiallergic therapy out of season that may abrogate the arrest of IDO-1 by decreasing the production of NO<b>∙</b>. Considering its association with the pathophysiology of atopic disease, tryptophan metabolism may play a relevant role in the pathophysiology of allergic disorders.