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To test the hypothesis that signet ring cell (SRC) histology has a negative prognostic value in patients with gastric adenocarcinoma (ADC).In western countries, gastric ADC with SRC often occurs after the disease has advanced. Consequently, the prognosis of SRC is generally regarded as poor, although survival studies comparing SRC and non-SRC have yielded inconsistent results.An intent to treat analysis was performed among 215 patients with gastric ADC scheduled for surgical resection from 1996 to 2007. Of these, 180 patients underwent the resection and 35 were not resected due to diffuse metastatic illness. From 59 resected patients with SRC (SRC group), control non-SRC resected patients matched by age, gender, American Society of Anaesthesiologists (ASA) classification, tumoral location, and pTNM stage were randomly selected by computer (non-SRC group: n = 100) during the same study period.The overall median survival was 21 months, which was significantly higher in resected compared to non-resected patients (31 vs. 5 months, P < 0.001). In non-resected patients, SRC histological subtype was associated with higher rates of diffuse peritoneal carcinomatosis (90.1% vs. 62.5%, P = 0.053) and neoplastic ascitis (63.6% vs. 34.7%, P = 0.059) and poorer median survival (5 vs. 7 months, P = 0.062). For resected patients, the 2 groups (SRC and non-SRC) were comparable regarding matching variables, demographic variables, and postoperative course. The median survival was significantly lower for SRC patients (21 vs. 44 months, P = 0.004). SRC resected patients exhibited higher rates of localized peritoneal carcinomatosis (P = 0.013) and lymph node involvement (P < 0.001) at diagnosis, lower R0 resection rate (P = 0.019) and earlier tumor relapse (P = 0.009), which was generally in a peritoneal carcinomatosis form (P = 0.011). By multivariate analysis, we concluded that SRC histology was independently associated with a dismal prognosis after adjustment on confounding variables (hazard ratio = 1.5, 95% confidence interval 1.1–2.0, P = 0.004). The prognostic role of SRC was maintained after exclusion of patients with advanced stage at initial diagnosis such as localized peritoneal carcinomatosis or lymph node invasion.This study is currently the best evidence showing that SRC is a major and independent predictor of poor prognosis due to specific characteristics such as more infiltrating tumors showing affinity for lymphatic tissue accompanied by a higher rate of peritoneal carcinomatosis. Our results suggest the need for a specific therapeutic strategy for such tumors.