PURPOSE: In patients with malignancies, thrombocytosis has been related to the disease stage and survival rate. In this study, special interest was directed to the possible prognostic significance of the platelet counts at diagnosis in patients with gastric cancer. METHODS: Three hundred and sixty-one patients with histologically confirmed gastric cancer who were underwent surgery at the Department of Surgery, Hanyang University Hospital, between January 1994 and December 1998 were reviewed. The relationship between the platelet count (PLT: 10(9)/L), the total WBC count (10(9)/L), the hemoglobin concentration (Hb; g/mL), the serum albumin (g/liter), the disease progression, and the survival rate of the patients were investigated. RESULTS: Thrombocytosis (PLT greater than 400) was present in 29 patients (8.0%) and anemia (Hb less than 12) was present in 130 patients (36.0%). The mean value of the PLT count increased significantly according to the advancement of the disease stage (stage I; 242, stage II; 258, stage III; 277, stage IV; 288, P value=0.000). The hemoglobin concentrations were negatively correlated with the platelet counts (gamma=-0.257, P=0.000). The prevalence of thrombocytosis was 3.0% and 8.3% in patients with stages I and II, respectively, which increased to 10.8% and 12.9% in patients with stages III and IV, respectively. These differences in the prevalence of thrombocytosis reached statistical significance (P=0.022). The five-year survival rate in patients with or without thrombocytosis was significantly different (42.6%, 65.9%, respectively; P=0.0123) CONCLUSION: Because patients with thrombocytosis have correlation with advancing stage and a poor prognosis, the platelet count at diagnosis may give additional prognostic information for patients with gastric cancer. Elevated platelet counts are mainly associated with the presence of anemia.