Type 9 T‑helper (Th9) cells are associated with atopic and inflammatory diseases. Their increased levels and functions contribute to a number of inflammatory disorders, where they are accompanied by enhanced Th2‑cell activity. However, there is currently no consensus regarding the association between Th9 and Th2 cells. Th9 cells may be induced from naïve T (Th0) cells under specific polarization conditions in vitro, a process driven by the addition of specific cytokines. In the present study, Th0 cells were cultured under Th9‑polarizing conditions to promote differentiation into interleukin (IL)‑4+ IL‑9‑or IL‑4‑IL‑9+ T cells after 3 or 5 days in culture, respectively; the mRNA expression levels of IL‑9 and IL‑4 were consistent with the induced cell types. Simultaneously, the levels of interferon‑regulatory factor 4 (IRF‑4) and Smad3/Smad4 were significantly increased following Th9‑cell polarization. It was therefore proposed that Th2 cells may be generated in the early stages of Th9‑cell differentiation, and then ultimately differentiate into Th9 cells via the Smad3/Smad4 and IRF‑4 activation pathway.