<b><i>Background:</i></b> T helper (Th)17 cells may play a role in allergic asthma. This study assessed the effect of allergen inhalation challenge on circulating Th17 cells and related cytokines in allergic asthmatics. <b><i>Methods:</i></b> Peripheral blood mononuclear cells were collected from 16 atopic asthmatics before and 24 h after allergen challenge, as well as from 10 atopic nonasthmatics and 10 normal controls. Cells were stained for Th17 cytokines and their receptors (IL-17A, IL-17F, IL-21, IL-22, IL-17R, and IL-23R) using flow cytometry. Cytokine concentrations from cell culture supernatants were quantified using a multiplex assay for IL-17A, IL-17F, IL-21, IL-22, and IL-23. <b><i>Results:</i></b> At baseline, asthmatics had a higher percentage of circulating Th17 cells (1.2 ± 0.5%) compared to normal controls (0.9 ± 0.66%, p < 0.001) but not compared to atopic nonasthmatics (1.13 ± 0.5%). There was a significant increase in Th17 cells in asthmatics after allergen challenge to 1.55 ± 0.4% (p < 0.05) and a trend toward significance in IL-17R expression from 3.4 ± 4.3 to 6.86 ± 6.84% after allergen challenge (p = 0.06). There was also a significant reduction in IL-21-positive cells following allergen challenge from 3.46 ± 1.85 to 2.33 ± 1.37% (p < 0.001). There were no significant differences in IL-17F, IL-22 and IL-23R expression. The concentration of IL-17A in culture supernatant was significantly higher in asthmatics compared to normal controls and IL-17A significantly increased 24 h after allergen challenge. <b><i>Conclusions:</i></b> The increase of Th17 cells and IL-17A in atopic asthma after allergen inhalation challenge suggests a possible role for Th17 in allergen-induced airway responses.