Alterations in transforming growth factor-beta signaling, due to a decrease in Smad2 and especially Smad4 expression, has primarily been reported in pancreatic and colorectal cancers, although loss of the chromosomal region 18q21.1, containing the loci of Smad2 and Smad4, is among the most frequent molecular alterations in cervical cancer. The aim of our study was to investigate whether decreased Smad2 and Smad4 protein expression in primary cervical cancers is associated with molecular alterations at 18q21.1, mutations in the functional domains of Smad2 and Smad4 or hypermethylation, and to assess the biological relevance of decreased Smad2 and Smad4 expression. Subsequently, Smad2, Smad4 and p21 protein expression was determined by immunohistochemistry in 117 primary cervical carcinomas, assembled in a tissue array. Smad signaling was shown to be associated with p21 mRNA expression. All the tumors expressed Smad2 or Smad4. Weak cytoplasmic Smad2 or weak cytoplasmic Smad4 expression could not be attributed to loss of heterozygosity at 18q21.1. Despite weak/moderate Smad2 expression and absent nuclear Smad4 expression, the coding regions of the functional MH1 and MH2 domains of Smad2 and Smad4 were unchanged, as assessed by sequence analysis. The Smad4 promoter region was unmethylated in tumor samples with weak/moderate cytoplasmic Smad4 expression. Remarkably, both weak cytoplasmic Smad4 expression and absent nuclear Smad4 expression significantly correlated with poor disease-free (P=0.003 and P=0.003, respectively) and overall 5-year survival (P=0.003 and P=0.010, respectively). Our findings support the hypothesis that Smad4 is a target molecule for functional inactivation in cervical cancer.