Transforming growth factor (TGF)-β1, as a candidate tumor marker, is currently of interest. In this study, serum TGF-β1 levels in gastric cancer (GC) patients and healthy volunteers were measured using enzyme-linked immunosorbent assay (ELISA). In addition, single nucleotide polymorphisms (SNPs) of the TGF-β1 gene at codon 10 and codon 25 were identified by means of amplification refractory mutation system–polymerase chain reaction (ARMS-PCR) and sequence analysis. Our results indicated that serum concentrations of TGF-β1 in GC patients were significantly higher than those in the control, and positively correlated with tumor mass, invasion, metastasis, and clinical stage. The serum TGF-β1 levels of patients recovering from radical resection were markedly lower than those before surgery. Meanwhile, no deoxyribonucleic acid (DNA) sequence variation at codon 25 of the TGF-β1 gene was found and a TGF-β1 gene polymorphism at codon 10 did not show obvious correlations with either TGF-β1 expression or clinicopathological parameters of GC. Our evidence suggested that serum concentration of TGF-β1 might be a novel tumor marker for GC and the polymorphisms of TGF-β1 gene did not play a role as a determinant of serum TGF-β1 concentration or as a genetic risk factor in the gastric carcinogenesis and progression. J. Clin. Lab. Anal. 22:164–171, 2008. © 2008 Wiley-Liss, Inc.