Background: Transforming growth factor beta(TGF-beta) signaling functions as a suppressor or a promoter in tumor development, depending on the tumor stage and type. However, the role of TGF-beta signaling in nonfunctioning pituitary adenomas (NFPAs) has not been explored. Methods: TGF-beta 1, Smad2, phospho-Smad2 (p-Smad2), Smad3, phospho-Smad3 (p-Smad3), Smad4, and Smad7 were detected in 5 cases of normal anterior pituitaries, 29 cases of invasive NFPAs, and 21 cases of noninvasive NFPAs by real-time quantitative reverse transcription polymerase chain reaction (qRT-PCR), Western blot, and immunohistochemical analysis. Results: The Smad3 and p-Smad3 protein levels gradually decreased from normal anterior pituitaries, noninvasive NFPAs, to invasive NFPAs. However, there were no significant differences in Smad2 (P = 0.122) and p-Smad2 protein levels (P = 0.101) or Smad2 mRNA level (P = 0.409). In addition, the TGF-beta 1 mRNA level gradually decreased while the Smad7 mRNA level gradually increased from normal anterior pituitaries, noninvasive NFPAs, to invasive NFPAs. Furthermore, proliferating cell nuclear antigen (PCNA) mRNA level was markedly increased in invasive NFPAs compared to noninvasive ones (P < 0.01), and its level was negatively correlated with Smad3 mRNA level (P < 0.01). Conclusion: The activity of TGF-beta signaling may be restrained in NFPAs and is correlated with the development and invasion of NFPAs.