Several proteins, including transforming growth factor β (TGF-β)receptor type I (RI), TGF-β receptor type II (RII), Smad2, Smad3, and Smad4/DPC4, have been identified in the transduction pathway of the tumor suppressor TGF-β. Mutations in TGF-βRI, TGF-β RII, Smad2, and Smad4/DPC4 genes are associated with several human cancers. The present study examines these gene mutations in 32 human ovarian cancers and 14 patient-matched normal tissues. For the first time, mutations in the Smad2 and Smad4 genes were analyzed in relation to human ovarian cancer. Gene mutations of TGF-β RI,TGF-β RII, Smad2, and Smad4were analyzed using specific primers by PCR-single-strand conformational polymorphism (SSCP), and the results revealed a frameshift mutation at codons 276–277 (CTCTGG→CTGCGTGG) in exon 5 of TGF-β RI in 10 of 32 tumor samples(31.3%). This mutation was associated with reduced or absent expression of TGF-β RI protein and p53 protein in tumor tissues. We detected SSCP variants of TGF-β RIIin exon 2 in 20 of 32 tumors. Sequence analysis of these variants revealed an A to G transition at the seventh band of intron 2. In this A to G polymorphism in intron 2, 12 samples (37.5%)had A/A alleles, 12 (37.5%) had A/G alleles, and 8 (25%) had G/G alleles. We detected Smad2 SSCP variants in exon 4 in 12 of 32 tumors (37.5%). Sequence analysis revealed a 2-bp deletion in the polypyrimidine tract of intron 3, which is located at position −39 to −56 in the splice acceptor site of the intron 3-exon 4 junction. No SSCP variants were detected in the Smad4 gene. These findings suggest that mutations in the TGF-β RI and in its signal transduction pathway are likely responsible for human ovarian carcinogenesis.