In cancer patients, the balance between neutrophil (N) and lymphocyte (L) cell counts fluctuates with advancing disease. The objective of our study was to determine the prognostic implications of the N/L ratio in the peripheral blood of gastric cancer patients. Study participants were identified from a prospective cohort of patients with advanced gastric cancer in Japan (n= 1,220). The median baseline N/L was 2.58 (range, 0.63–12.7). Univariate analysis revealed that patients with an N/L≧ 2.5 (n= 644) had a significantly poorer prognosis than those with an N/L< 2.5 (n= 576; log rank test, p= 0.019× 10–12). The median survival times for these two groups were 239 (95% confidence interval, CI, 217–251 days) and 363 days (95% CI, 334–406 days), respectively, while the 1-year survival rates were 30 (95% CI, 26–34%) and 50%(95% CI, 45–54%), respectively. A multivariate Cox model established a significant relationship between the N/L ratio and survival (adjusted hazard ratio= 1.52; 95% CI, 1.32–1.75; p= 0.077× 10–8). These results suggest that the N/L ratio is an independent prognostic factor in advanced gastric cancer. Measurement of this ratio may serve as a clinically accessible and useful biomarker for patient survival.