The aim of this study is to describe and relate nutritional and inflammatory status and prognostic indexes with postoperative complications and clinical outcome of patients with gastrointestinal malignancies. Twenty-nine patients were evaluated; nutritional assessment was carried out by subjective and objective parameters; albumin, pre-albumin, C-reactive protein (CRP), and alpha-1-acid glycoprotein (AGP) were determined. To assess prognosis, the Glasgow scale, the Prognostic Inflammatory Nutritional Index (PINI), and CRP/albumin ratio were used; the clinical outcomes considered were hospital discharge and death. A high Subjective Global Assessment (SGA) score was associated with the occurrence of postoperative complications: 73% of the patients with postoperative complications had the highest SGA score, but only 6% of those without postoperative complications had the highest SGA score (P < 0.001). Greater occurrence of death was observed in patients with a high SGA score, low serum albumin, increased CRP, PINI > 1, and Glasgow score 2. There was a positive correlation between weight loss percentage with serum CRP levels (P = 0.002), CRP/albumin (P = 0.002), PINI (P = 0.002), and Glasgow score (P = 0.000). This study provides evidence that the assessment of the nutritional status and the use of prognostic indexes are good tools for predicting postoperative complications and clinical outcome in patients with gastrointestinal neoplasia.