Transforming growth factor-beta1 (TGF-beta1) plays an important role in the pathogenesis of multiple malignancies, and also, its expression strongly affects the outcomes of cancer patients. The objective of this study was to determine the clinical significance of the serum levels of TGF-beta1 in gastric cancer patients. A total of 63 patients with a pathologically confirmed diagnosis of gastric cancer were enrolled into this study. Serum TGF-beta1 concentrations were determined by the solid-phase sandwich ELISA method. Thirty healthy age- and sex-matched controls were included in the analysis. The median age at diagnosis was 62 years, range 28 to 82 years. There was no significant difference in baseline serum TGF-beta1 levels between gastric cancer patients and the healthy control group (p = 0.08). The known clinical variables including age of patient, gender, site of lesion, histology, histological grade, stage of disease, and serum levels of lactate dehydrogenase (LDH), CEA, and carbohydrate antigen (CA) 19.9 were not found to be correlated with serum TGF-beta1 concentrations (p > 0.05). However, the chemotherapy-responsive patients had higher serum TGF-beta1 levels compared with chemotherapy-unresponsive ones (median values 330.50 v 49.54 pg/mL, respectively, p = 0.01). Moreover, patients with elevated serum TGF-beta1 concentrations had significantly favorable overall survival compared with those with lower levels (median 71.1 v 39.9 weeks, respectively, p = 0.04). In conclusion, serum levels of TGF-beta1 may have predictive and prognostic roles in patients with gastric cancer.