It was suggested that there are no stronger prognostic factors in gastric cancer than nodal involvement or depth of wall invasion. The present paper evaluated the influence of maximum tumor diameter (MTD) value, measured on fixed resected specimens, on the extent of disease progression and the outcome in gastric cancer patients. Clinicopathological data were retrospectively retrieved from records of 122 patients who underwent curative gastrectomy. The patients' MTD values were grouped as follows: MTD1, up to 26 mm; MTD2, between 26 and 50 mm; and MTD3, over 50 mm. The three groups evidenced significant differences with regard to 5 year survival (MTD1: 54%, MTD2: 31%, MTD3: 20%; p = 0.00027), furthermore they were significantly different with respect to the type of gastrectomy (p = 0.021), depth wall invasion (p = 0.000), lymphatic microinvasion (p = 0.014), perineural microinvasion (p = 0.017), stromal reaction (p = 0.025), and stage (p = 0.035). ROC curve analysis individuated a best accurate MTD threshold value for nodal involvement of 32 mm (sensitivity = 56.6%; specificity = 60.9%; positive predictive value = 52.6%; negative predictive value = 64.6%). The logistic regression analysis suggested that the depth of wall invasion was the only independent variable associated with MTD value (p = 0.0005). Multivariate analysis showed that independent prognostic risk factors were sex (p < 0.0025), number of involved nodes (p < 0.001) and MTD (p < 0.001). In conclusion, the maximum tumor diameter value of gastric cancer may be a factor with greater prognostic implications than previously believed.