Studies have suggested that glutamine synthetase (GS) is a potential marker of hepatocellular carcinoma (HCC). We aimed to evaluate the expression of GS in non-malignant liver tissue and serum GS levels in HCC, liver cirrhosis (LC), chronic hepatitis B (CHB), five kinds of extrahepatic diseases patients and healthy subjects. Immunohistochemistry (IHC) was used to assess GS expression in 260 liver tissue samples (from 120 HCC, 90 CHB stage 4, and 50 CHB stage 1-3 patients). Enzyme-linked immunosorbent assays of 325 samples (from 100 healthy donors, 33 CHB stage 1-3, 43 CHB stage 4, 111 HCC, and 45 extrahepatic diseases patients) were used to further analyze GS levels in serum. IHC studies showed the expression of GS in 70% of HCC patients, 46.7% of CHB stage 4 patients and 38% of CHB stage 1-3 patients. The chi(2) tests showed significant difference between HCC samples and non-tumor tissues (P = 0.001 for HCC vs. CHB stage 4, P = 0.000 for HCC vs. CHB). Consistent with this, serum GS levels are increased in HCC and CHB stage 1-4 patients. There are significant differences among all samples (P = 0.000 for all), except CHB stage 1-3 versus CHB stage 4 (P = 0.552). Based on multiple linear regressions, HCC, CHB stage 1-4 and AFP were significantly associated with serum GS levels. In addition, in HCC group, TNM and Child-Pugh were significantly associated with GS levels. Expression of GS is increased in HCC, LC, and CHB. It may be a new serum marker for liver disease.